Last week I took an Amtrak train south for the holiday weekend, and there was a delay. The explanation: the track was under a 'heat advisory' and the train had to travel at a slower speed. This left me wondering what happened to the tracks when it got so hot outside - and why they couldn't travel at speed.
|An Amtrak passenger train. Image Credit: modified from Massachusetts Office of Transportation on Flickr.|
American tracks tend to be continuous welded rail, which means that the quarter-mile steel tracks are welded together and tied down along the way. Train tracks used to have expansion joints (like bridges) that allowed them to move more in the heat and cold. But longer tracks means trains can travel faster and smoother, give less friction between the train and the tracks, and are easier to maintain. Because the tracks don't have a lot freedom to move, they're tied down at high temperatures (95 - 100°F) to try and reduce the amount of expansion in the summer heat.
When the rails are already heated by weather, the added energy from the train traveling over the tracks can be enough to cause them to become too hot. Since the rails are restricted from expanding along their length, they expand in the next easiest direction: out.
|Examples of extreme rail buckling. Image Credit: US DOT and Volpe National Transportation Center|
Amtrak doesn't own all the rails it travels on, so it has to follow the heat advisories of the companies that own the track. Most of my trip through Virginia was on tracks owned by CSX. When it's over 90°F, CSX regulations dictate that passenger trains have to travel 20 mph under their normal speed. Since it was 100°F on the day I was traveling, we were well in the range of having to slow down. So while the trip was longer than I wanted it to be, I'll take a delay over a train derailment any day.