The third rail, on many light rail systems such as subways and elevated trains, is mounted above and to the side of the two rails which support the train's weight.

The third rail is mounted on plastic insulating supports, and is usually covered by a plastic shroud which prevents track workers from accidentally contacting it.

Power is supplied to the trains through the third rail. While the voltage may vary between different systems, it is usually around 800 volts DC. This is supplied by mercury arc or silicon rectifiers off of the AC power grid. As I witnessed once in a rather cool malfunction on Miami's Metrorail system, there is enough current present at the third rail to sustain the arc and melt down the iron rail (estimated failure current in the THOUSANDS of amps) in the event of a short circuit! Ever seen a waterfall of molten iron?

The two other rails used in a third-rail system are connected to both earth ground and neutral on the power system, and serve as a return path for the power used by the train. Sometimes, third rail and overhead wire systems may be combined. Power is picked up from the third rail by a sliding metal contact on the side of the train cars, which sort of resembles a duck's foot. The sliding action of the contact on the third rail probably helps keep the iron contact surface from rusting and thus becoming less conductive.

The third rail system is not without its problems. First, it allows unauthorized, or, authorized persons walking the track to come in contact with the 800 volt DC source, with expectedly catastrophic results. Second, the contact on the side of the train can sometimes get borked up, leading to truly awesome fireworks. Watch for it when riding at night or in a tunnel!

The London Underground uses a fourth rail in addition to the third. The fourth runs between the two running rails and serves as the negative (as opposed to the train's wheels like in streetcars, most third rail trains, and your model railroad).

There are a number of disadvantages to third rails in addition to VXO's 'what if you step on it?'. The first will be familiar to anyone who's ever ridden a subway - the lights going out! It's impossible to maintain a complete third rail all the way around a switch, so while your subway car's contact shoe isn't touching the third rail, all the lights go out (along with the air conditioner).

The other is speed limitation. Your typical third rail system would have a maximum speed of about 100 mph or so - anything more and you'd start damaging both the contact shoe on the train, and the third rail itself. Granted for most light rail systems this won't be a problem, but where full-size trains employ a third rail this is limiting for speed.

Third rails produce awesome blue sparks not normally seen with overhead catenary (or maybe it's just because it's dark in the tunnel). They even can be employed for streetcars in the form of a trough running between the rails under the street.

Most of the sparks you see coming from a train in a dark tunnel are the result of a subway car's contact plate losing contact with one section of powered rail and picking it up on the next. The powered rail becomes vertically thinner as it comes to its end, and goes from thinner to thicker as it starts again. Sometimes the shoe grates against the beginning of the rail as it makes contact with it, causing a spark.

This happens most frequently on switches because of two factors - first because there's no way to run a continuous third rail along a switch (the rail would cut diagonally along the running rails) necessitating a break in the powered rail, and secondly because the train is making contact with the new third rail at an angle instead of straight on.

This is also why the sparks you see are periodic - it happens every time a new car passes the point where the new third rail starts.

One seeming problem with the third rail system would be the loss of power that a train would experience when it crosses switches or otherwise loses contact with the third rail. However, this problem is easily rectified. The CTA solves this problem by having a battery system in each car. Thus, as the train passes across the gap between two parts of the third rail, the battery takes over, keeping the lights on. The only noticeable change for passengers inside the train is the fact that the blowers for the heating or air conditioning switch off temporarily. It is only if a train is stopped over a gap in the third rail, or if, on a curve, the train loses contact with the rail for an extended period of time, that the lights will shut off.

Here in Chicago, there are tools called "stingers" to help trains stranded in just these types of situations. The stingers are basically live wires with insulated handles that can be touched to the contact shoe on the train to give it a momentary burst of power, enough to get the train back onto the third rail. Stingers are seen at common lost-power points, such as switches, sharp curves, and level crossings.

Another complaint about third rails is the danger of electrocution involved in using them. However, this is not as much of a problem as many may think. First off, transit systems put a lot of work into keeping people off of the tracks, regardless of whether or not a third rail is used. Anyone who does manage to make it on to the tracks also puts himself at risk of being hit by a fast-moving train. Secondly, on elevated train lines, it is very hard to electrocute yourself on the third rail. This is because the rail sits on glass insulators, and the tracks themselves sit on wooden ties. Thus, someone touching the third rail doesn't get electrocuted because she isn't grounded. However, if someone touches both the third rail and one of the remaining two rails, closing the circuit, or manages to ground himself, then he will be electrocuted.

Rain can also help to close a circuit in a third rail system. Normally, the ceramic insulators and wooden ties are enough so there is no danger of the system shorting out in any way. However, the constant rubbing of contact shoes and wheels on the rails causes small bits of rust to come off. Some of this dust settles on the glass insulators and wooden ties, and eventually these get coated with a light layer of grime and rust. This isn't a problem normally, as the rust-grime combination is a poor conductor, but when it rains, the water is enough to cause a circuit to form from the third rail, through the rusty grime on the insulators and cross ties, and down the metal superstructure to the ground, or in the case of grade level tracks, through the grime directly to the ground. The result is a very low-grade short across the entire line. Thus, it isn't uncommon to see CTA workers standing or sitting on the third rail when it's dry (on the elevated tracks only, of course), but as soon as the first drops of rain fall, the workers rush off of the tracks.

Third rail. (Electric Railways)


The third rail used in the third-rail system.


An electric railway using such a rail.



© Webster 1913

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