Purpose of fourth rail on tracks

Sometimes on subway tracks you see a fourth or even fifth rail sitting between the two running rails. Pictured here: http://imgur.com/a/vFWz8" onclick="window.open(this.href);return false;

Why are those extra rails present?

Are they spare rails stored in situ or are they there for structural or safety reasons?

One source suggests they prevent derailed trains from going too far away from the tracks: http://ask.metafilter.com/59277/What-is ... h-rail-for" onclick="window.open(this.href);return false;

From the picture you posted those are guard rails, which help prevent a derailed train from traveling to far from the running rails. That photo is of elevated trackage (el), so they would prevent a long drop to the street below, as long as it is a low speed derailment. Guard rails are also scene on curved trackage and switch points.

There are time when spare rails are stored between the running rails and they are usually spiked at the ends of the rail, where as the photos shows spikes on all of the ties. Making it a guard rail.

JoeD

Hmm interesting. Those guard rails appear to have a slightly different shape from the running rails. Is that from lack of wear or are the rails different?

How long has it been since a train on elevated tracks derailed in the NYC system?

Not sure. I've scene guard rails being of a lighter weight, 100 lb/yd as compared to 120 lb/yd. Also i have scene the recycling of of worn running rail.

The New York City elevated is not my strong suit. I do remember two elevated accidents, 9th Av Elevated on September 11, 1905 and Brooklyn Elevated June of 1923. I believe both ended up on the street. I am sure someone will have better info. I don't count the Malbone Street accident since that was in a tunnel but did involve wooden el cars.

JoeD