How A Streetcar Works

71/Watertown-Harvard streetcar and 70/Watertown-Central trackless trolley at old Watertown Square Station in 1957.

The first and foremost fact one should know about streetcars is that they are electric. Since the late 19th century, streetcars (and later trackless trolleys) have run along main roads powered solely by electric overhead wires. These wires are supported over streetcars and trackless trolleys by trolley poles, many of which remain along streets today from since-terminated trolley lines (see "Trolley Remnants" under articles for more on remaining poles).

Look at the photo above. You should see two orange MTA vehicles, on the left a PCC streetcar and on the right a Pullman-Standard trackless trolley. Both are drawing their power from overhead wires; however, the way the streetcar does so is practically different from the way the trackless trolley does. This difference stems from the fact that streetcars have metal wheels that run along steel rails, and trackless trolleys, like buses, have rubber tires that run on regular asphalt roads.

Because streetcars travel along steel rails with metal wheels, the circuit between the overhead wires and a streetcar's electronics is grounded. For those not familiar with electricity, a circuit must be grounded with a path to ground so as to prevent electrocution of surrounding creatures. For streetcars, the path back to the ground is through the wheels to the rails; therefore, streetcars require only one overhead pole, as you should see in the photo above, to draw electricity from the wires above.

Trackless trolleys, as shown above, require two overhead poles in order to be properly powered. This is because of their rubber tires; if electricity were to travel through those tires to ground, then the tires would be fried by the electricity, which is supplied at 600 Volts DC to power a typical trolley! Quite a bit of electricity! Therefore, trackless trolleys require a second wire, which is not powered, to ground their power circuit; hence, trackless trolleys cannot run on streetcar wires and require a second overhead ground wire alongside their overhead power wire. The second trolley pole on a trackless trolley runs along the ground wire and grounds the circuit.

Trackless trolley trolley poles running along double wires, one for power and one for ground, strung specifically for trackless trolleys.

Streetcars, on the other hand, since they are grounded through their wheels can run on trackless trolley wires; along San Francisco's Market Street, for instance, streetcars run back-to-back with trackless trolleys on the same wires. The streetcar just attaches its trolley pole to the powered wire, and it runs just fine.

PCC in Waverley Square in Belmont in 1958 using the powered wire of a wire pair put in place due to Route 73's impending trackless trolley conversion.

While the poles trackless trolleys use to draw power from overhead wires have remained unchanged to this day, the way streetcars do so has changed rather significantly over the years. While older streetcars used trolley poles as trackless trolleys do today, newer streetcars generally use pantographs:

Pantograph on a Green Line Type 8 streetcar. In contrast, all of the streetcars and trackless trolleys pictured earlier in this article have had trolley poles.

Pantographs, as pictured above, are wide metal platforms that slide along the bottom of the overhead wire to draw power from the wire. In contrast, trolley poles clamp around overhead wires, ensuring they maintain contact with the wire (and therefore remain powered) by harnessing the wire within a socket at the top of the pole. The socket on the top of the pole has a bottom and two sides, and is raised to clamp around the wire and slide along it as the streetcar travels:

Trolley pole clamped around overhead wire on a PCC in San Francisco, CA.

Because a pantograph does not need to stay clamped around the wire to draw power from it and is wide enough to maintain contact with the wire even with slight sideways movements that occur around bends, the pantograph is capable of handling travel at higher speeds than a trolley pole can. Passing along curves at high speeds with trolley-pole equipped vehicles could easily result in "de-wiring," as the trolley pole is fixed to a bottom turntable that spins slowly to pass along a curve. Likewise, another advantage of a pantograph is less moving parts, as it is fixed to the ceiling of of the trolley in one position, meaning that less can go wrong with the pantograph.

Even so, a pantograph can only handle the slight sideways movements of a trolley fixed to a track—the pantograph could not handle the lane-weaving of a trackless trolley as it is not wide enough. Also, the pantograph cannot physically move from side to side as a trolley pole can, as it is fixed in one position at the center of the trolley's roof, meaning that changing lanes would be impossible without de-wiring. Therefore, pantographs are only used on streetcars, such as the Green Line trolleys (though not on the Mattapan PCCs, which still use trolley poles) and not on trackless trolleys such as the 71, 72, 73 and 77A.

Another advantage of pantographs over trolley poles is that using pantographs eliminates the need for wire switches when a trolley passes through a track switch. To clarify, when a track splits into two directions, a track switch is used to switch the direction the trolley will go from one route to another:

Track switch at the abandoned yet still intact streetcar trackage on Walley Street in East Boston, located in the busway of Suffolk Downs Station on the Blue Line. Coming from Waldemar Avenue, streetcars could either continue straight across the switch to enter Gladstone Loop, or turn left to continue to Suffolk Downs Loop.

In order to control which direction the streetcar takes at the switch, the operator of the streetcar either pulls power through the switch to go one way or coasts through the switch to go another. Alternatively, a switch operator stands by the switch and, depending on the streetcar's rollsign, presses a button to electrically switch the switch.

Track switches are also used to turn back streetcars early on short-turn routes, such as the E Line's occasional early termination at Brigham Circle, where there is a track switch to reverse the streetcar from an outbound track to an inbound one or vice versa. Only double-ended streetcars can reverse direction at switches, as the operator must move to the opposite end of the streetcar to control it from there once the streetcar has been reversed. Single-ended streetcars, such as the Mattapan PCCs, require a loop to reverse direction.

Track switch at Coolidge Corner Station on the C Line in Brookline, looking west. The switch is currently set to allow trolleys to pass through it—if you look at the switch curve hitting the track on the left, you will see that there is a gap to allow trolley wheels to pass by the curve. Were the curve touching the left rail, trolleys coming west along the track (in the wrong direction servicewise) would be transferred to the right rail as their wheels would follow it, and trolleys traveling east would hit the curve and to be able to pass. Note the overhead wires—there is an additional wire along the switch curve so that the pantograph remains in contact with wire as the trolley switches and the trolley remains powered.

In order to ensure that a streetcar remains powered while it switches, additional trolley wire is suspended over the switch as seen in the photograph above. Pantograph-equipped streetcars require no additional infrastructure to remained powered while they switch; the top base of the pantograph simply continues to pass along the wire over the switch and loses contact with the main wire. However, for trolley-pole equipped streetcars, an additional wire switch is required to pass through a switch.

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The wire switch at the intersection of Mount Auburn Street and Belmont Street in Cambridge. Here, the 71 and 73 trackless trolleys, which previously ran together along Mount Auburn Street, split, the 71 to continue along Mount Auburn Street to Watertown Square, and the 73 to merge onto Belmont Street and continue to Waverley Square in Belmont. As mentioned previously, trackless trolleys use trolley poles; therefore, they require the wire switch shown above to continue along their respective routes.

Trolleys pass through wire switches similarly to how they pass through track switches. To go in one direction, the trolley coasts through the switch, and to go in the other the trolley pulls power through it. Wire switches are sometimes called "frogs," as they resemble a frog with its legs outstretched.

Only continuous (trolley pole) wires require wire switches; dead-ended (pantograph) wire does not require a switch as the pantograph will simply glide along the wire in the direction it is traveling at a fork as explained earlier. To clarify, there are two types of trolley wire setups that are generally used. The first and more classic type is continuous wire—this is the type of trolley wire that Boston used on the Green Line until it stopped using PCC streetcars in 1985 and continues to use today on the 71, 72, 73 and 77A trackless trolleys. Continuous wire basically involves suspending a very long, live overhead wire from trolley poles all along a trolley route. 

The disadvantage to continuous wire is that it must be re-tensioned regularly—over time, the wire begins to sag, due in part to the copper the wire is made of expanding and contracting, and must be tightened so that trolleys can draw power from it. For this reason, a second wire type, dead-ended wire, is used on the Green Line today. Dead-ended wire does not require re-tensioning—in its base state, it is not tight, and nor is it one long wire. Rather, the dead-ended wire is made up of many wire sections about 1/4 of a mile long hung from a second wire above the power wire. Weights hang from the end of each wire section, suspended from the top wire, and by means of these weights the trolley passing along the wire causes enough tension to line the wire sections up with one another—at every junction of wire sections there is a pulley that brings the sections in line once there is enough tension caused by the weighing down of the wire.

PCC streetcars in Mattapan Yard. As you can see, the Mattapan PCCs use trolley poles, and the entire Mattapan Line is equipped with single continuous overhead wire, as shown by the red arrow. Image copyright Me (Gil Propp).

Type Eight streetcar traveling along the C Branch of the Green Line on Beacon Street in Brookline. As you can see, Green Line streetcars use pantographs, such as the one circled in green, and the entire Green Line is equipped with dead-ended overhead wire, as shown by the red arrow. As circled in blue, pulley-equipped weights hang along the wire, bringing the dead-ended wire into line as the streetcar approaches and puts pressure upon the dead-ended wire. The weights and wire sections hang from the top wire, and the streetcar draws power from the bottom wire. Image copyright Me (Gil Propp).

Since trolley pole-equipped streetcars use continuous wires, they are specifically outfitted to clamp around the wire and draw power from the wire by sliding along it continuously. Therefore, trolley pole-equipped streetcars cannot use dead-ended wire. Accordingly, the Mattapan PCCs cannot run on the Green Line; if one of them were to be run on the Green Line, it would have to be outfitted with a pantograph like 3295, the PCC parked at Boylston Station. However, pantograph-equipped streetcars, which merely require constant contact with the wire, can use continuous wire; the MBTA could, if it so desired, run Type 7 and Type 8 LRVs along the Mattapan Line, which is equipped with continuous wire.

While continuous wire allows for the possibility of suspending a second overhead wire for grounding, such as with trackless trolleys, pantograph (dead-ended) wire, where the wire travels across the streetcar's pantograph's wide top platform in a zigzag motion, does not allow for such infrastructure. Therefore, it would be impractical to use pantograph wire for a trackless trolley, as trackless trolleys require a second overhead wire for grounding as mentioned earlier—the pantograph on the top of the trolley is very wide and leaves no room for a second pole or wire to be practically attached for grounding purposes, not to mention that a zigzagging pantograph would bump into the second wire and cause additional problems.

On a streetcar, however, where the vehicle travels using metal wheels as opposed to the trackless trolley's rubber tires, suspending a second wire for grounding is not an issue because, as mentioned earlier in this article, the streetcar is already grounded through the wheels and the tracks! While rubber trackless trolley wheels simply glide along asphalt roads like with any other typical vehicle, metal streetcar wheels travel along metal tracks that dictate the streetcar's path. Because both the wheels and the tracks are made of metal, together they keep the streetcar grounded.

Unused trolley wheels at the Shore Line Trolley Museum in Connecticut, a Streetcar Tracks shooting location. As you can see, trolley wheels are made entirely of metal, with no wooden component whatsoever, as they are used for grounding the streetcars power supply in addition to moving the streetcar. Note the grooves in the wheels—streetcar wheels are built such that they clamp around the racks they travel along, thereby ensuring a constant, secure contact with the metal tracks below and grounding the streetcar. Image copyright Journey Thru My Lens Photo Blog (Link)

Installed but never used streetcar tracks at Forest Hills Station in Jamaica Plain. These tracks would have served the Arborway Branch of the Green Line's E Branch were service restored along it (see "Trolley Remnants" and "What Happened to the Arborway Line?" for more information). As you can see, the tracks are shaped such that trolley wheels can fit securely onto the tracks and travel along them in a smooth matter while ensuring that contact with metal is maintained, thereby keeping the streetcar electrically grounded. Image copyright Me (Gil Propp).

Streetcar tracks also provide crucial infrastructure for Boston's streetcars to brake. There are two main types of streetcar brakes—disc brakes, which are applied to the wheels by increasing pressure on the brake shoes, thereby stopping the streetcar by applying pressure to the streetcar's wheels, and track brakes, which stop the streetcar by applying pressure to the tracks. In most cities, streetcars primarily use disc brakes, with track brakes serving as emergency or parking solutions when additional stopping force is needed. Boston, a city with many hills and variable weather conditions, uses track brakes on its streetcars much more often than most other cities, as disc brakes often cannot provide reliable enough force to stop streetcars on Boston's steep terrain (think South Huntington Avenue in Mission Hill on the E Line) or when it is raining or snowing.

Circled in red is the air compressor on a Type Three Boston streetcar (to learn more about streetcar types, visit "Trolley Types of Boston"). In earlier streetcars, disc (or friction) brakes would be applied to the wheels by applying air pressure, which was generated in and routed from the air compressor, onto the brake shoes. Some modern streetcars still use air pressure to apply such pressure, yet nowadays some, such as the modern Boston Type Eights, use springs. Air compressors tend to be very loud, hence the loud, high-pitched noise associated with Type Fives and older Boston streetcars.

Official diagram of MBTA Type 7 LRV by Kinki-Sharyo. Circled in blue are the Type 7 LRV's three track brakes. Track brakes on Boston's streetcars are electromagnetic—the brakes are first lowered onto the tracks, and then current is sent through electromagnetic coils in the brakes that cause the brake to stick firmly to the track. Image copyright Kinki-Sharyo.

Dallas PCC on Heath Street Loop along the Green Line's E Branch in Mission Hill. Circled in blue is the streetcar's electromagnetic track brake; the brake is lowered onto the track, creating friction between the brake and the track that stops the streetcar. Image courtesy Boston Transit Library.

Boston's twisty streets, which in many neighborhoods resemble the opposite of a grid plan, are hard for longer streetcars to navigate. A longer streetcar would likely derail on some of Boston's tighter curves (again, think South Huntington Avenue on the E Line). Considering that Boston's streetcars are heavily ridden, it is necessary for rolling stock to be articulated if many riders are to be accommodated at once.

Articulation involves placing flexible joints and a rolling floor plate at the center of the streetcar so that the streetcar, with its long wheelbase, can maneuver sharp curves. Any longer vehicle has trouble turning along sharp curves, and a streetcar is no exception—longer buses require articulation as well. With articulation, the vehicle is eased along the curve, making sure that it does not drift over into neighboring lanes or hit nearby objects while it turns.

Boeing LRV rounding the bend along Prendegast Avenue in Brighton between Reservoir Yard and Cleveland Circle in 1979. As you can see circled in red, Boeing LRVs were articulated and had a flexible joint at their centers that would move back and forth with the motion of the streetcar, just as the joint of this one has "curved" the streetcar along Prendegast Avenue. Image copyright Mike Szilagyi.

Image to come shortly of the center interior rolling plate on articulated streetcars and buses.

As a testament to Boston's sharp turns, the Boston Elevated Railway was not only the first transit agency to use articulated transit vehicles, but also the inventor of the articulated streetcar! In the early 1910s, seeking to expand its streetcar system's passenger capacity, BERy took two converted horsecars and joined them together with a flexible joint in the middle, thereby creating a first-of-its-kind articulated center-entrance car! Nickname "snake cars," these streetcars were plagued with problems and were accordingly taken out of service by the mid-1920s. However, the idea of articulation was a good one for Boston, and fifty years later articulated streetcars became the standard in Boston, starting with the Boeing LRVs pictured above and persisting to this day with the modern Type 7s and Type 8s.

Original BERy "snake car," built from two converted horsecars connected by a center flexible joint. Passengers could enter the car through a door in the center joint or through doors at either end of the streetcar.

Train of articulated Type 7 and Type 8 streetcars on the outbound E Branch of the Green Line and articulated bus on Route 39, both along Huntington Avenue in Boston by Northeastern University. The flexible joints at the centers of each vehicle are all circled in red—note that the modern joints are made of plastic "accordions" rather than of metal. Image copyright Me (Gil Propp).

Note that there are no couplers on the "snake car"—coupling of streetcars, or joining together several streetcars and operating them together as a train, began to be used on Boston streetcars in the 1910s with the advent of the center-entrance car, a high-capacity streetcar built specifically to handle large crowds of riders (see "Trolley Types of Boston" for more information). Coupling functions as an alternative to articulation, allowing streetcars to be joined together to increase passenger capacity while also ensuring that the long train can round the sharp curves of the streets of Boston.

Three-car train of "Post-War" PCC streetcars in Cleveland Circle in Brighton. The train is currently rounding the curve between the end of the Beacon Street streetcar reservation used by the C Branch of the Green Line and Reservoir Carhouse, a pretty sharp curve. As you can see, the train is curved into a half-circle; look at the area circled in blue, and you will see that the streetcars are connected to one another (with couplers) yet are clearly not in a straight line. The couplers allow the long train of streetcars to easily round the curve while not drifting into neighboring lanes or derailing; as shown by the coupler circled in red above, the couplers can move from side to side, functioning as articulators. Compare the above coupler to the ones in the next few photos, and you will see that the couplers are at very different angles with respect to the center of the cars they are on. Image Copyright Lew Schneider.

Center-entrance cars and Type Fours were the first streetcars in Boston to use couplers, and accordingly two or even three-car trains of center-entrance cars, Type Fours or both were a common sight along the streets and streetcar subways of Boston from the 1910s through the early 1950s.

Three-car train of center-entrance cars passing through Kenmore Square in the 1920s. As you can see, the cars themselves are not articulated, yet they are attached to one another with couplers, such as the one visible at the front end of the lead car, ensuring that they can make it around the sharp curves of Boston's streets and streetcar subways Image courtesy Boston Public Library.
(For a photo of a Center Entrance-Type Four combination train, and for more information on these streetcars, see "Trolley Types of Boston")

In addition to functioning as articulators, couplers later also handled as data transferrers between the cars they coupled. Functions such as radio, door control, wipers, lights and announcements were and still are all transferred between streetcars via couplers, ensuring that the head operator could operate both cars at once with ease. Look at the photograph below; the PCC pictured is an "All-Electric," the first streetcar in Boston to control all of its functions (doors, wipers, etc.) electrically as opposed to mechanically. Accordingly, all-electric PCCs were the first streetcars in Boston where the head operator could control all streetcars in the train electrically—in trains of all-electrics, command data was transferred between streetcars through their couplers.

Look at the circled coupler—since this is an "All-Electric" PCC, the coupler functions as both a streetcar connector (left of connector) and a data transfer path (small round connectors on right of coupler). Image Copyright Me (Gil Propp).

Between being controlled electrically and mechanically, the basic way streetcar doors, and bus and trackless trolley doors for that matter, work has not changed significantly over the years. The path is simple—the doors have straight guiding paths at their top and bottom and have pegs that are inserted into these paths and slide along the paths to open and close the doors. The doors are connected to their right and left panes with hinges so that, when the pegs slide along the top and bottom guiding paths, the doors fold onto the sides of the door frame, ensuring that riders have a wide way through which to enter or exit the streetcar.

Stopped "Post-War" PCC streetcar, specifically one of the last streetcars ever to run on the Arborway Branch of the Green Line's E Branch (note tape over "Arborway" sign on the left, signaling the end of service on the line), stopped at Park Street (for more on the Arborway Line, see "What Happened to the Arborway Line?"). As you can see, the streetcar's doors are wide open—the doors have folded onto the side of the streetcar, leaving a full doorway through which to enter and sit the streetcar. Look closely, and you will see that the at the center of the doorway (on the outer end of each door half) there is a peg that inserts into tracks at the top and bottom of the door frame, which together with hinges on the doors ensure that, when the doors are folded open, they fold to the side of the door frame and vice versa.

Originally, the doors would be opened and closed by varying air pressure to push doors open and closed (applying high pressure pushes doors closed, releasing pressure opens doors). Nowadays, an electronic mechanism moves the door pegs along their track in order to open and close the door. Overall, from doors to trolley poles to articulation, the basic way a streetcar works has not changed—what has changed is technological refinement as technology evolves and design is modernized and improved.

Type Eight streetcar on the Green Line's C Branch using a pantograph, a modern adaptation of the trolley pole that better accommodates movement, particularly from side to side, at high speeds, to draw power from overhead wires.

Trackless trolleys in Waverley Square in Belmont using the same trolley poles that have been used since the first streetcars hit the streets of Boston in the 1880s. Since trackless trolleys move between lanes on city streets, they are not compatible with pantographs and therefore must be used with trolley poles.