Why do they need a third rail in electric train systems?

Why do they need to install a third rail in electric train systems? Why can’t they just electrify one of the two rails that are already there?

(Obviously they CAN do electric trains with overhead wires as well… I’m not talking about those systems.)

The existing rails are grounded via the crossties, so they wouldn’t be very useful for running high voltage through. This is why the third rail is raised a few inches above the ground. Also, it would make things difficult to properly insulate the rails in places where there are track switches. Also also, current between the rails in sections of track (via the train axle) is used by the signalling systems to tell when a train is on a particular section of track, and electrifying the rails would mess that up. (And every non-electric train would short out the two rails via its solid axle.)

What friedo said. All good reasons to have a third rail. Additionally, grade crossings might present a few issues with pedestrians as well as other traffic.

a) Track switches could be insulated by having gaps in strategic places, and the momentum of the train will carry it over such gaps. This is done, by the way, when the third rail encounters a switch track.

b) Non-electric trains? I haven’t heard of a third-rail system sharing the rails with diesel-electric trains…only those systems with overhead wires tend to share the rails. Is there a system which uses a third rail AND shares the rails with non-electric trains? Very uncommon, if it exists at all, no? - Jinx

It would be exceedingly dangerous. For one thing, the body of the train is grounded and so trains could go only in one direction (think about it) on any given stretch of track. Also if you look at most third rails, you will see that they are constructed so that accidental touching is very unlikely. You are touching the train and accidentally brush the live track. Bye bye. Grade crossings would be a nightmare (already are even for conventional third rail. Catenaries are far better, although a well-designed third rail works well in most situations.

Metro-North, Long Island Rail Road and Amtrak all have combined diesel and electric service in certain places.

…And I forgot to mention, the NYC subway’s various work and construction equipment are mostly diesel powered.

The Long Island Rail Road does this on the western portions of the track. In fact, the most western portions are shared with third rails (LIRR electric trains), catenaries (Amtrak & New Jersey Transit) and desiel trains (LIRR).

I believe Metro-North does this on the southern portions of its tracks, too.

Metro-North shares its third-rail powered Harlem Valley and Hudson Division lines with diesel powered Amtrak trains and construction equipment. The northern portion of the New Haven Division line and branches are caternary powered but does not, to my knowledge, carry any regular diesel traffic.

SmackFu was this question precipitated by chance by watching “Mythbusters” last night on TDC? If so, I just thought I’d let you know that Cecil had a different take:

just imagine the nightmare you’d have whenever it rained if they electrified one of the wheel rails.

The last time I rode the Long Island Railroad it was third rail electric and then steam from there out to Westbury. Of course that was about 1955…

Yep! You got me. Intersestingly, they link back to the Straight Dope on their web page, along with Snopes.

For a two-rail system you need to insulate half the wheels on all trains, and I think that would be way too difficult and dangerous. You can’t have solid axles - every single axle and half the bearings would need to be insulated, and there will be numeraous places in the train’s undercarriage where there’s a full line voltage (several thousand volts) across small gaps.

There’s also the danger of having an exposed live wire lying an inch or two above the ground, and the difficulty in insulating it. Any object dropped on the live rail would cause a short circuit. Even a bit of rain might do it.

The UK rail network uses an electrified thrid rail; most passenger services are electric, most freight services are diesel-electric (all on the same set of rails)