Tuesday, July 08, 2014

Expect Delays: Why Trains Slow Down When It's Hot

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.
Most railroad tracks are metal and when (most) metal heats, it expands. As the temperature rises, the molecules in the metal vibrate more, and they need more 'space,' so the volume increases. Different materials expand at different rates when they're heated. That's why if you have a glass jar with a lid you can't open, you can run it under hot water. The lid will expand more than the glass - allowing you to easily open the container.
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
As temperatures rise and the tracks heat up, train companies want to reduce the amount of stress the rails are under. The load the train puts on the track (vertically) is dependent on the speed of the train, the diameter of the train's wheels, and the weight that the train is carrying. To change the amount of vertical load on the track, the diameter of the wheels can't change, nor can the weight of the train (unless we start throwing passengers off the train), so the only thing to change is the speed.

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.


  1. It's nice that someone took the time to understand what was going on rather than bitch at the fact that the train was late. Great right up.

  2. Very interesting. Something new I've learned today. :)

  3. Can you please tell me if this has been a factor in any of the bomb train derailments? Are the oil train companies taking precautions for this? I've been concerned about the Bakken crude oil (much lighter, more volatile and explosive than regular crude) trains, with their 100+ train car loads and the fact that they are already hard to stop, take curves, etc. This is the first time I've thought about the additional risk of heat-related train rail deformities. Thanks!

  4. Yes thanks so much to explain this. I am on Amtrack near DC right now and they slowed..actually stopped as it is over 90. So it was helpful to learn more by reading more details of the physics of this!