Double rails on bridges?

[Maury Markowitz]

I'm sure the answer to this is commonly known to true rail fans... but why do railways running on bridges almost always have a second set of rails running inside the main ones? They do not appear to be load bearing, and often seem only haphazardly connected. I assumed they are additional safety in case of a derailment, but the examples I've seen up close seem too flimsy to be effective.

[D40LF]

It is called a check rail and it is to keep any derailed cars from falling off the bridge. You can also get them inside tunnels.

[rollsign29]

I've also seen check rails near significant structures or trackside objects, such as a building or even an overhead bridge, but they're not as common. I think check rails on bridges are a design requirement rather than an option.

[M. Parsons]

On 06/07/2016 at 5:40 AM, Maury Markowitz said:

and often seem only haphazardly connected. I assumed they are additional safety in case of a derailment, but the examples I've seen up close seem too flimsy to be effective.

In a lot of cases it's reused rail that's been ripped up from somewhere else. On Edmonton's LRT bridge, the guardrails date from pre-1950, despite the bridge being built in the late 1980's and the running rail laid in the early 1990's. 

[Maury Markowitz]

So I seem to be missing one key point here: a derailment takes a certain amount of energy, I assume, so if there's enough for that to occur how is this old and barely connected rail supposed to stop the derailment if the original rails didn't?

[smallspy]

3 hours ago, Maury Markowitz said:

So I seem to be missing one key point here: a derailment takes a certain amount of energy, I assume, so if there's enough for that to occur how is this old and barely connected rail supposed to stop the derailment if the original rails didn't?

Because the kinetic energy in a railway derailment is primarily focused on the Z axis (fore and aft), not the X axis (side to side). It doesn't take nearly as much force to restrain a train from going sideways as it does to stop it.

 

Remember, the only point of the check rails is to prevent a derailment that's stayed within the gauge (one wheel between the rails) from getting beyond a certain and specific point - say a bridge girder or a tunnel wall. The running rails will also help in that situation as well.

 

Dan

[dbdb]

31 minutes ago, smallspy said:

That description of the forces and energy needs a free body diagram!  The physics (dynamics) of a train on rails is quite interesting and a more complex than the simple arrangement of a flanged wheel on parallel rails would suggest, especially at speed.

 

[Bus_Medic]

One already did.

[Maury Markowitz]

On 2017-09-29 at 1:45 PM, smallspy said:

Because the kinetic energy in a railway derailment is primarily focused on the Z axis (fore and aft), not the X axis (side to side). It doesn't take nearly as much force to restrain a train from going sideways as it does to stop it.

 

Sure, but it had to have enough energy in X to get off the main rail in the first place. Lifting the entire train enough so the flange can get all the way over the rail seems like a MUCH more difficult thing to do than push a barely connected rail sideways.

[smallspy]

24 minutes ago, Maury Markowitz said:

Sure, but it had to have enough energy in X to get off the main rail in the first place. Lifting the entire train enough so the flange can get all the way over the rail seems like a MUCH more difficult thing to do than push a barely connected rail sideways.

Maybe yes....but maybe no. How did it come off the rail in the first place? Impact with vertical forces? Rolled rail? Other lateral forces?

 

The other thing that you may be missing is that in most cases any derailments over a bridge occurred well before the bridge. The equipment was able to run in the gauge for some distance before reaching the bridge. In that event, there aren't any more longitudinal forces (at least until the brakes are applied) and thus the check rails are there to prevent striking damage to the structure.

 

Look, it's certainly not a perfect device. But considering that they cost next-to-nothing in the grand scheme of things, and don't require any maintenance, it seems like an easy decision to make in order to keep your plant in good shape.

 

Dan