Third Rail - Benefits and Disadvantages

Benefits and Disadvantages

Electric traction systems (where electric power is generated at a remote power station and transmitted to the trains) are considerably more cost-effective than diesel or steam units, where the power unit is carried on the train. This advantage is especially marked in urban and rapid transit systems with a high traffic density.

So far as initial cost is concerned, third-rail systems are cheaper to install, compared to overhead wire contact systems, as no structures for carrying the overhead contact wires are required, and there is no need to reconstruct overbridges or tunnels to provide clearances. There is also much less visual intrusion on the environment.

However as third rail systems present electric shock hazards, higher system voltages (above 1,500 V) are not considered safe. Very high currents are therefore used, resulting in power loss in the system, and requiring relatively closely spaced feed points (sub-stations).

The presence of an electrified rail also makes it hazardous for a person to fall into the tracks. This can be avoided using platform screen doors or the risk minimized by ensuring that the conductor rail is on the side of the track away from the platform.

Furthermore, third rail systems have the risk of pedestrians from walking onto the tracks at level crossings. In the U.S. a 1992 Supreme Court of Illinois decision affirmed a $1.5 million verdict against the Chicago Transit Authority for failing to stop an intoxicated person from walking onto the tracks at a level crossing and attempting to urinate on the third rail. The Paris Metro has graphic warning signs pointing out the dangers of urinating on third rails; which Chicago did not have.

The end ramps of conductor rails (where they are interrupted, or change sides) present a practical limitation on speed due to the mechanical impact of the shoe, and 160 km/h (99 mph) is considered the upper limit of practical third-rail operation. The world speed record for a third rail train is 174 km/h (108 mph) attained on 11 April 1988 by a British Class 442 EMU.

Third rail systems using top contact are prone to accumulations of snow, and ice formed from refrozen snow, and this can interrupt operations. Some systems operate dedicated de-icing trains to deposit an oily fluid on the conductor rail to prevent the build-up.

Because of the gaps in the conductor rail (e.g. at level crossings and junctions) it is possible for a train to stop in a position where all of its shoes are in gaps, so that no traction power is available. The train is said to be "gapped". In these circumstances a following train is brought up behind the stranded train to push it on to the conductor rail or a jumper cable is used to supply enough power to the train to get one of its contact shoes back on the third rail. This gives a minimum length of multiple unit on a line. Locomotives have either had the backup of an on-board diesel engine system (e.g. British Rail Class 73), or connected to shoes on the rolling stock (e.g. Metropolitan Railway).