When riding Metro in Washington, I noted that the third rail shoes run on top of the power rail. But I recall that there are also properties that used shoes under the rail (under-running). What is the reason for the different design? Are any properties still using the under-running rail?

Wesley

I believe the Grand Central Terminal (NYC/Metro North, etc) electrification is still under running 3rd rail, while Penn Station's 3rd rail (PRR/LIRR) is over running.

The under running sytem had the advantage of being immune to leaves and other debris accumulating on the running surface.

New Haven's FL-9 locomotives were originally equipped with pneumatically adjustable/retractable 3rd rail shoes, capable of operating on either system, or retracting up out of the way.

Jeff

Interestingly, a small portion of the ex-New York Central West Side line, built by the pioneer of "over-running," is now electrified via "top-running" on approach to Penn Station, as Amtrak's dual mode units are set up for the LIRR system. For this reason, the occasional Amtrak reroute into Grand Central dictates an escort by a Metro North unit equipped with the proper third rail shoes.

It's always been my feeling that the underdog "under-running" is an inherently safer design, although both have been responsible for their fair share of incidents -- most, of course, being entirely preventable and the mere result of human stupidity. A girl in my friend's high school class was electrocuted on the LIRR near Garden City a few years ago after she stumbled while taking the "shortcut" back from a party. Go figure.

In addition to Metro North, the other system using underrunning third rail is the Market-Frankford rapid transit line in Philadelphia. It has not been installed on any new system since before 1910.

Although underrunning third rail is somewhat safer, it is also more difficult to install, being suspended from a top support. For this reason, it will fall down in situations that would not affect overrunning third rail.

Incidentally, modern plastic overrunning third rail guards are much safer than the older board designs in that they enclose the sides of the rail as well as providing a cover over the top.

MBTA in Boston and CTA in Chicago still use unprotected overrunning third rail, which dates from the 19th century and is the only type of third rail feasible with the early gravity third rail shoes, as opposed to the more common spring loaded shoes.

Michigan Central probably used underrunning shoes between Detroit, Mich., and Windsor, Ont.

I have in my mind that the reason for both was a commercial or patent issue. Could one have been Westinghouse supply and the other GE?



Underrunning requires insulators in tension instead of compression. So just setting the rail on top of a simple porcelain insulator doesn't cut it.

Another problem with underrunning is that when a support fails and the third rail drops down, it immediately breaks off every third rail shoe on that side of every passing train until repaired.

Does anyone know of a Museum with third rail installed? (Not necessarily electrified) I know IRM has talked for years about putting third rail at the 50th Ave. "L" station to show the unique Chicago gravity link shoes in (mechanical only!) operation, but has not yet installed it.

We've talked about installing a demo third rail at WRM. However, given the California Public Utilities Commission's orders about totally fencing such installations it would never be energized.

We just finished building an automatic overhead/third rail changeover switch for Sacramento Northern interurban 1005, so the car will be third-rail capable if necessary.

Having grown up and knowing some railroads in the New York area, I always understood that the great advantage of the NYC's underrunning third rail was during ice storms. The LIRR and the Transit Authority would get all tied up with ice on the rail - broken shoes, spectacular shorts, trains without a shoe making contact, while the New York Central's trains continued to run.

Branford Electric Railway Association/Shore line Trolley Museum has installed some used 3rd rail ties from Staten Island Rapid Transit Operating Authority. They do have the sockets for mounting an insulator, but nothing more has been done about any demmonstration.

Underrunning third rail certainly seems like the winner from a technology perspective. Because in the modern age, you are absolutely obliged to cover it for safety.

To cover an overrunning third rail requires an entire structure be built above the third rail, cantilever by necessity, supported from only one side! Nominally this must support the weight of a man falling on it. So we're talking about a stout structure, whose falling down would damage passing trains. Seems like at that point you might as well just have under-running, and wrap a simple insulator directly around it.

I'm sure BART did extensive studies of this. I wonder where you'd research that?

I once read about an active electronic safety system for third rails. It was a system that would monitor the current being drawn from a section of third rail, and if the current went over a threshold (beyond normal leakage), yet was not as much as a locomotive would draw, the section power was shut down in a split second. Was this ever used? On paper it seems quite reasonable - limit a 600 Volt shock to a few hundred milliseconds, and the victim will very likely survive with only a painful burn he will never forget.

Still, probably not good enough for a museum environment.

As a side question - what is normal leakage current for a section of third rail?

--
Will

The problem with under running trolley rail is you have to support the entire weight of the rail, plus resist the forces of thermal expansion and also the forces applied to the rail by the shoes sliding on it. Then, you still have to suspend the insulated guard above that, since an air gap between the guard and the rail is a much better insulator than having the insulation touching the rail, since dirt and rain water could form a conductive current path on the surface of whatever was used for insulation.

On the other hand, over running trolley rails set up for use with flipper type shoes only have to have C shaped brackets to support boards above and along the field side of the rail.

I personally didn't have a problem with Chicago's open trolley rail. It kept the riffraff off the tracks, and the riffraff was much more dangerous than the trolley rail.

What about a side running (horizontal force) third rail?

http://www.irm.org/gallery/subexpansion

IRM's "new" substation. Note photo 8, the "Type 76 DC overcurrent relay"...

According to an article in the summer 2008 Rail & Wire, that piece of equipment was purchased for a "not inconsiderable sum". It sniffs the current flowing to the trolley wire, looking for anomalous current draws. I don't believe it's intended to detect the scenario you describe, but perhaps it's not as farfetched as you think... nor is its possession by a railway museum.

The electric lines out of Birmingham, England (MU commuter cars only) used a side-running third rail; I'm not sure if this system is still in use. Not aware of any other installation.

On another subject, the current required to electrocute a person is no more than that needed to light a string of light bulbs, so it's impossible for the substation to try to determine whether someone is in fact getting a shock.