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After I graduated with a degree in Mechanical Engineering I went to work for FreightMaster, a company that manufactured end-of-car cushioning units for railcars. The units are huge shock absorbers that are installed behind railcar couplers to help cushion the “lading” (i.e, the stuff being transported). We would put strain gauges on the shafts of these shock absorbers and smash railcars together to test new designs. Very cool.
FreightMaster also sold simulators used to train locomotive engineers. A simulator consisted of a minicomputer attached to a control panel from a diesel-electric locomotive. A monitor connected to the computer told you how fast you were going, what your brake line air pressure was, whether you’d just gone off the rails, and so on. As part of the deal, FreigtMaster would “digitize” the routes that the engineers would someday drive and feed the routes into the simulator.
I did a bit of programming for the system and got to play a locomotive engineer. In the process, I found out how easy it is to derail a train. The most obvious way to do that is to take a curve too fast and let inertia do the rest. But there are more subtle ways to make it happen.
For example, if you accelerate on a curve, you put the train – which is like a long, stiff string – in tension. Too much tension, and you pull the string off the inside of the curve. Conversely, if you brake while in a curve, you put the string in compression. Too much compression and you push the string off the outside of the curve.
The strangest way to derail a train is via “resonance.” Resonance occurs when all the oscillations in a system reinforce – rather than offset – each other so that the system oscillates at a higher amplitude. If you’ve ever seen a video of the Tacoma Narrows Bridge collapsing, you have an idea of what resonance can do.
Trains tend to “rock” on American tracks because of the way in which the rails are laid. Rails are 39’ long so that they will fit inside a 40’ gondola car. When rails are laid, the joints are offset by half a rail, so that there is a joint on the track every 19.5’ – first on one rail and then on the next. The weight of the trains traveling over the joints causes them to “work,” and they end up being a bit lower than the rest of the track. As the cars roll over the joints, then, they dip a bit. This means that the cars “rock” as they dip first to one side and then to the other.
At 17.5 mph, the cars hit resonance. If they stay in resonance long enough, they can actually hop off the tracks. Therefore, the engineer must pass through 17.5 mph – whether he’s accelerating or decelerating – as quickly as possible.Published in