On May 26, 2009, at 20:29 , Bob Sullivan wrote:
There can be 6 inches of slack per car in a train. This can help you start a train rolling as you stretch out the accordion one car at a time. Remember the locomotives only weigh so much and the coefficient of friction of steel wheels on steel rail is only .03 or so (even if you're sanding the rail). Getting going can be tough. And being in the 100th car back can give you a bad case of whiplash as the momentum of 99 cars moving at 1/2 mile per hour accelerate you to 1/2 mile per hour instantly! But the slack can also play havoc with the train over the road. Imagine a slight grade, followed by a dip, followed by another grade and a 100 car train of loaded coal hoppers. The engines strain to take you over the first grade and you stretch out all the slack on the way up. But on the way down, you compress the slack out of the train and then play 'crack the whip' on the way up, pulling the slack back out. If you're lucky, you don't break a coupler and make 2 train segments. Regards, Bob S.
Probably does not happen much anymore without a defect or a rapidly arising situation, given that the modern engines are controlled digitally, and that control is, among other things, modified by the digital strain gauges that are located throughout the construct.
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