A friend of mine recently asked, in response to the Railroad Crossing post, why unused crossings are labeled with EXEMPT instead of just being removed. It all goes back to fail-safe.
First, the purpose of the exempt sign is to notify buses and semi-trucks they are not required to stop at the crossing as it is no longer used by trains. Remember those old days when your school bus would stop at a crossing, the driver would open the door, wait, close the door and then proceed? Exempt signs merely state this is not required and the bus may blast through at normal speed.
Semaphore at MN State Fairgrounds Exhibit in the Stop aspect. The weight of the semaphore blade will pull the signal to this position if the rod should break.
Second, a fail-safe is a method or practice that maintains safety even though a particular mechanical or electrical component may malfunction. In other words, it will “Fail Safely.”
With railroads, any malfunctioning equipment has a fail-safe putting it in the most restrictive aspect when malfunctioning. This is usually done by gravity since it never fails and is always in abundant supply.
Let’s take crossing arms for example. When not in use, they are actually held up by a mechanism. When they are engaged the motor turns off and gravity allows the arms to drop. (Actually, according to Matt’s Web site below, there is a motor that helps the arm fall during the first portion of the decent, but they should still be able to fall naturally if the motor cannot hold them up during a failure). This is an example of fail safe because in the event of a power outage, the arms will drop.
The old semaphores also worked this way. They were weighed in such a way where if the rod that changed their aspect broke, gravity would pull the arm down into the stop aspect. Also, if a railroad signal light (those traffic-like signals next to the tracks) is out it is treated as a stop and a train needs special clearance from a dispatcher to proceed.
Let’s return to the EXEMPT sign. If the exempt sign were to fall off (here gravity comes into play again), the crossing would be treated as any other active crossing. Buses and trucks would stop which really does no harm except slow them down. It would be more tragic if in standard practice the crossing signs were just removed to show an exempt status. Imagine a wind storm or wild vandalism spree removing an active crossing sign. The absence of the sign, with the tracks still running through the intersection, would cause an unsafe situation as drivers would expect the track to be exempt. True practice, since rails are usually expensive to remove, is to keep the crossing signs there and add an EXEMPT sign. And yes, this also means that if you approach a crossing with rails through the cement but no signs, you should observe it as if there were warning signs there.
For more information on signaling, I recommend the book Railroad Signaling by Brian Solomon. A supurb detailed online reference about how signals work would be Matt’s Web Page: Signal Department.
