Maybe you've seen one of those old movies in which a train engineer spies trouble ahead and slams on the brakes. Scene: Camera focuses on spinning train wheel, brakes lock up, wheel slides along the rail, sparks fly all over the place.
That's an interesting picture, but it doesn't happen in this day and time.
Every few months the TV news bombards us about another wreck in which a train plowed into a motor vehicle. Yesterday it was in New York.
Two years ago there was one here in Midland, Texas, where a Union Pacific train hit a parade float killing some people.
That case is currently in litigation, and the plaintiffs' cases appears to hinge on the theory that the engineer had time to stop before the train struck the float. But since the judge wouldn't allow a recording of the engineer's utterance upon seeing the first float cross the track -- which the plaintiffs' lawyer contended should have been enough to convince the engineer to begin slowing the train --- the defendant filed a motion for summary judgment which is currently being considered by the judge. All of this comes to us by way of the morning paper which has covered the accident extensively. See the latest article.
Anyway, this post is about how a train stops. A long train traveling at around 60 MPH will need about a mile to stop. And currently the most popular braking system is one developed by George Westinghouse generations ago using air pressure.
The brakes are "on" by default. Pads squeeze against the wheel, and air pressure is used to push a piston which pushes the pads off the wheels so they can turn. And the train engineer uses controls in the engine to regulate the air pressure which either activates or deactivates the brakes. See howstuffworks.com.
Here's what seems to be a flaw in that system. The air pressure starts at the engine and goes back. So when the engineer activates the brakes, the air pressure is bled off the front brakes first, and the process proceeds toward the back of the train. The result is that the wheels at the rear of the train get their brakes activated last, and there's an accordion effect as the rear cars try to pass the slower cars in front. In a worst case scenario, the train could derail. (This information comes from a real train engineer speaking about railroad crossing safety.) Naturally, an engineer doesn't want to slam on the brakes.
A train has the right of way on any railroad track, and size matters in vehicle collisions. So it's best to get out of the way of a moving train. Now if only they had an anti-locking brake system to work all wheels at the same time.
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