Railroads Solved Big Problems With A Little Physics.
One of the remarkable things about trains and train travel is the fact that modern railroads are still using technology that’s 150 or so years old.
Certainly, there have been refinements like computers in the locomotive cabs and the complex system of signals that keep trains from running into each other. But essentially, trains today pretty much work the same way they did back during the Civil War. Someone, years ago, came up with some wonderfully simple solutions to some pretty knotty problems.
Slowing and stopping trains has always been a concern.. After all, it’s massive weight moving rapidly and with a great deal of momentum. For years, air pressure was used to apply the brakes to the wheels, and that worked fine … unless an air hose broke somewhere along the length of the train. Loss of air pressure meant no brakes at all. Toward the end of the 1800s, George Westinghouse had what seems today to have been an obvious idea: reverse the whole concept and use the air pressure to keep the brakes off the wheels. That way, if an air hose broke, instead of suddenly having no brakes, all the brakes would be applied and the train would stop safely.
But how does a train start? How does the locomotive manage to get a 10,000-ton freight train moving? Answer: It’s because of the slack between each rail car that’s built into the coupling mechanism. As the locomotive starts moving the slack is taken up and the train starts moving one car at a time. And that the locomotive can handle.
Here’s my favorite: Unlike just about all other rolling vehicles, a train’s wheels don’t revolve on the axel. Instead, they’re fastened to the axle so the two wheels and the axel turn as one unit. OK, that’s interesting, but it led me to another more complicated question: If both wheels are fixed to the axle and are turning at exactly the same rate, even I can understand that as the train goes around a curve, the outside wheel has to travel a somewhat greater distance than the inside wheel. So how come it doesn’t skid along the rail to keep up?
The answer is so simple it’s wonderful. Take a look at the bottom of the wheels in this illustration. Notice that the part of the wheel touching the rail doesn’t sit flat on the rail; it’s been fashioned at an angle. (It’s also been exaggerated here to help illustrate the point.) As the train travels through a right hand curve, centrifugal force pushes each rail car to the left. That causes the outside wheel to have a slightly greater diameter where it rests on the rail than does the wheel on the inside of the curve. And that means the outside wheel covers that small extra distance through the curve with the same number of revolutions as the other wheel. And no skidding.
Maybe it’s just me … but I think that’s really cool!