Extra rails

March 16, 2005 6:53 PM

Take a look at this picture. What are the extra rails for?

I thought at first it was one track diverging into two with a weird switch setup, but that doesn't seem to be the case. Any railfans on here who can help?

posted by greatgefilte to Travel & Transportation (14 answers total)

I know nothing of rail, but my guess would be that they lock the wheels to the static rail, so that the train can't slide to one side or otherwise push the moveable rail out of position, since I imagine a train has enough force to move those switch rails :-)
posted by -harlequin- at 7:04 PM on March 16, 2005

Sweet jumps.
posted by Mean Mr. Bucket at 7:04 PM on March 16, 2005

I assume by the extras you mean the 4-tie-spanning rails sitting just inside the main rails close to the viewer. I think you're right - it's part of the 2-to-1 / 1-to-2 switch. Looks like these would "trap" the outer wheels of any passing car to force it to run all the way to the left or right, ensuring the wheels on the other side to head the proper direction at the split. The bends towards the ends indicate to me that they are meant to catch and guide wheels to run in between the short pieces and the main rails, from either direction.

Of course, the caption mentions, "Government employees may have falsified documents related..." - maybe someone altered the picture.
posted by attercoppe at 7:10 PM on March 16, 2005

Train wheels have a solid axle (no differential) and they (very elegantly) solve the problem of going around a corner by having tapered wheels. "Centrifugal force" shifts the wheels sideways so the wheel on the outside of the curve is, in effect, larger than the inside wheel. The outside wheel then goes farther with the same rpms, and there is no wheel slippage. It looks to me like the extra rails are to guide the wheel flange to prevent unwanted sideways movement (hopping off the rails) as the two tracks merge into one.


posted by weapons-grade pandemonium at 7:12 PM on March 16, 2005

Your assumption is probably right. That looks like a fairly standard set of Y-shaped points to me plus a wide-angle lens.
posted by cillit bang at 7:13 PM on March 16, 2005

If you are speaking of the short (approx 5 ft long) bent rails on either side of the frog (the area where the rails "cross" in the center foreground of the photo) the are called guardrails and are to prevent the flange from "picking" the point of the frog if the direction of motion is towards the viewer. They work by pressing against the back of the flange on the "outside" wheel, keeping the "inside" flange away from the point, to prevent derailments.
posted by pjern at 7:18 PM on March 16, 2005

By the way, I've seen a lot of places on railroads where there are "islands" formed by an extra pair of rails running for a while inside the regular track (no junction or anything special on the track at that point). I always assumed these are to prevent catastrophic runaway derails (i.e. trap the derailed wheels between the rails) on critical segments of the track (e.g. bridges). Anyone care to confirm or deny this?
posted by azazello at 7:54 PM on March 16, 2005

azazello: Precisely correct. Usually found on bridges, as well as in some tunnels, and other critical areas. Sometimes they will be put only on one side, if it is more critical a derail go only one direction, and nothing much valuable on the other side, but that's rare. They are almost always exactly as you describe, and common.
posted by pjern at 1:24 AM on March 17, 2005

cillit bang technical nit: The "points" on a railroad switch are the moveable sections down at the far end of the above mentioned photograph that move to select one route or the other. Note that is it possible to "split" a switch (run through one set against you, away from the viewer in the above photo) sometimes without any damage. The "frog" is the section where the rails cross, and the rails between the frog and the points are called "closure rails".
posted by pjern at 1:31 AM on March 17, 2005

VAE Nortrak - Railroad Frogs Supplier
posted by Dick Paris at 3:04 AM on March 17, 2005

solopsist: Sorry to disappoint you, but those are points where I come from.
posted by cillit bang at 3:46 AM on March 17, 2005

I saw some on some model railways once, and they made carriages or trains that had derailed jump back onto the rails when they rolled over them.
posted by gaby at 3:59 AM on March 17, 2005

cillit bang Your Wiki link describes exactly what I was saying, doesn't it? Or is this a UK/US thing?
posted by pjern at 5:39 AM on March 17, 2005

solopsist: it's a UK/US thing. They're points here instead of switches. I think switch is much more descriptive. But no one asked me.
posted by grouse at 8:15 AM on March 17, 2005