The parent link says “Railroads deal with thermal expansion by heating the rail prior to installation”.
reddit/etmidust wrote “I was a manager of track maintenance for a US railroad for a while. When they lay new rail, part of the process is a ‘heater car’[1]. This machine rides the rail and is basically a multi headed blow torch to heat the new rail to the ‘rail neutral temperature’(SFT). At this point the rail is not yet spiked down to the ties so that it has a chance to grow and expand. After the heater car some other rail cars called spikers follow to spike the new rail down. That is how they ‘pre-stress’ the rail. I am skipping some steps, but that is the gist of it.”
The parent link says “Railroads deal with thermal expansion by heating the rail prior to installation”.
reddit/etmidust wrote “I was a manager of track maintenance for a US railroad for a while. When they lay new rail, part of the process is a ‘heater car’[1]. This machine rides the rail and is basically a multi headed blow torch to heat the new rail to the ‘rail neutral temperature’(SFT). At this point the rail is not yet spiked down to the ties so that it has a chance to grow and expand. After the heater car some other rail cars called spikers follow to spike the new rail down. That is how they ‘pre-stress’ the rail. I am skipping some steps, but that is the gist of it.”
[1] https://imgur.io/tvCy9C2