The following is an introduction to the different types of transit vehicles which are supplied directly with electricity. It is not intended to be an exhaustive list of all possibilities, but mainly a description of the systems in use in Vancouver (trolleybus and SkyTrain) and one considered for use in Vancouver (light rail).
There are, of course, other vehicles which use electric motors which are supplied with electricity generated by an on-board power source, such as a diesel-electric locomotive (e.g. West Coast Express) and the experimental fuel-cell Ballard bus. These latter types are not described here.
The advantages of electric vehicles are as follows:
- more energy efficient
- electric motors are typically 90% efficient, as compared to internal combustion engines which are at most 30% efficient (see the article on energy efficiency for a more detailed discussion of the issues)
- electric motors can be used as generators to slow the vehicle – this power can be used by other vehicles (called “regenerative” braking) or simply dissipated as heat through resistors (called “dynamic” braking). In either case the use of friction brakes is reduced – brake linings tend to last at least twice as long.
- less polluting
- no direct emissions – this also means there is no possibility of fumes inside the vehicle
- uses a renewable energy resource – in British Columbia this is especially true, since most electricity is generated by hydroelectric stations
- better at accelerating and climbing hills
- electric motors can provide maximum torque at zero speed – in fact, they can provide significantly more than normal maximum rated torque for a limited time (e.g. twice as much for 10 seconds)
- by comparison, internal combustion engines do not develop full rated torque until they are at a higher speed, and they require mechanical slippage in order to start
- no mechanical transmission
- there is no noise when the vehicle is stopped (aside from the possible operation of an air compressor on some vehicles) – internal combustion engines must idle
- electric motors themselves do not make as much noise as internal combustion engines of equivalent power
A trolleybus is a rubber tired vehicle which has two poles mounted on the roof. The poles are used to contact two overhead wires which run parallel to the street. The outside wire is at ground potential and the inside wire is “live” – typically 600 volts DC, although some systems use 700 volts DC.
The ends of the poles have “trolley shoes” which have sides which guide the shoe along the wire. The part that makes contact with the wire is made of graphite, and wears out over time. This change of “shoes” is performed regularly at the garage.
There is a rope attached to each pole which can be used to lower and raise the pole. The other end of the rope is stored on a spool mounted on the back of the bus. Some trolleybuses have “automatic retrievers” which “reel the pole in” when a power failure is noticed. This will prevent a pole from springing up and possibly damaging the overhead if the trolley shoe comes off the wire.
Intersections of trolley lines require switches which guide the trolley poles to the correct wires. A switch is either in a straight through or turnout position. Multiple options can be handled by installing more than one switch. For example, to provide a straight through, left turn or right turn at an intersection one switch is installed back of the intersection to select a wire over the left turn lane, and another switch is mounted close to the intersection to choose between straight through and a right turn.
There are three common types of switches:
- Power On / Power Off: drawing current trips the switch in the predetermined direction. There is no standard direction in Vancouver – you must go by the “POWER ->” or “< -POWER” signs on the overhead, or else notice the white painted “teardrop” on the pavement ahead of the switch which indicates the direction taken with power on.
- “Selectric: a pair of contactors ahead of the switch determines if the bus is turning by the relative position of the shoes on the wire (if the bus is turning one shoe will be ahead of the other), and throws the switch if required.
- Fahslabend (not used in Vancouver): bus turn signals or a dedicated switch for the driver cause a radio transmitter on one of the trolley poles to send a signal to the switch indicating the desired direction of travel.
Vancouver’s trolleybus system uses regenerative braking to slow the bus from higher speeds, with air brakes being used for the last bit of deceleration before a stop.
There are more than 370 cities in the world with trolleybus systems. Approximately 100 of these are in the former Soviet Union. The following North American cities operate trolleybuses: Edmonton, Vancouver, Boston, Dayton, Philadelphia, San Francisco, Seattle.
Light rail vehicles are supplied power by a single overhead wire which is typically at 600 or 750 volts DC. The tracks themselves are at ground potential and complete the electrical circuit.
Most modern light rail vehicles use pantographs to contact the wire. Pantographs are simply a wide contactor supported by an “elbow” which pushes the contactor against the wire. Typically the overhead wire is made to zig-zag a little bit in order to avoid wearing out only one part of the pantograph. No switches are required on the overhead when pantographs are used.
There are also light rail vehicles (called streetcars or trolley cars in North America) which use trolley poles to contact the wire. This is exactly the same as one of the trolley poles used on a trolleybus, and thus switches are required to guide the trolley pole at intersections.
The term “light rail” is a broad one, and generally refers to any rail vehicle which is typically used in short trains or single vehicles, and uses overhead wires to supply power. The use of overhead wires allows light rail lines to be constructed on street, as well as on more standard railway right of way. In fact, this is one of the main strengths of light rail – it can be constructed more cheaply by running on street rather than in tunnel or on an elevated structure.
There are over 340 cities in the world with light rail systems, and numerous manufacturers of light rail vehicles exist. The following North American cities operate light rail systems: Calgary, Edmonton, Toronto, Baltimore, Boston, Buffalo, Cleveland, Dallas, Denver, Los Angeles, New Orleans, Newark, Philadelphia, Pittsburgh, Portland, Sacramento, St. Louis, San Diego, San Francisco and San Jose. For more information on light rail transit, check out the Light Rail Transit Association’s web page.
SkyTrain has elements of both light rail and subway/metro. It uses side mounted rails (one at +300V, one at -300V) and contactor shoes, similar to what is used on subway trains (typically subways use 600V – 750V on one rail and the track is ground). This design essentially rules out level crossings, and in fact the SkyTrain is (and must be) completely grade separated.
SkyTrain also uses automated trains – there are no drivers or attendants. This is quite uncommon. Even more uncommon, however, is the use of linear induction motors rather than standard electric motors. Linear induction motors pull the train along the track by reacting with a “reaction rail” mounted in the centre of the track. The gap between the vehicle and the reaction rail must be maintained to close tolerance. On the positive side, however, the train can now accelerate and decelerate regardless of wheel to rail adhesion! The use of linear induction motors also allows for smaller wheels and a lower floor (and hence lower platforms at stations).
SkyTrain stations are the length of a six car train, with each car being only 12.7 m (42 feet) long – approximately the same length as a standard bus. The small car length and steerable trucks allow the SkyTrain to make tight turns with a minimum of noise. Platforms are at the same height as the train floor, allowing quick boarding.
SkyTrain is a single source system currently produced by Bombardier. There is a similar system (but employing human “drivers” – employees that sit in a cab and monitor the performance of the train) in Scarborough, Ontario, Canada; a single track loop in downtown Detroit, Michigan, USA (the “Downtown Peoplemover”); a line recently opened in Kuala Lumpur, Malaysia; and a line planned in New York City to connect John F Kennedy airport with subway and commuter rail stations. The last two lines use or will use a larger Mark II car design.
Thanks to Derek Cheung and Ian Fisher for providing technical details. Some SkyTrain information is from BC Transit brochures.James Strickland (james @ perforce.com)