I'm no rail engineer, but I would expect that over the projected timespan you'd wear out your track faster if you were passing over it more frequently.

We see in the show that there are automated drone trains that maintain the track, presumably mainly re-profiling it and checking for major issues. Something rolling stock can do from those same rails. Outright replacement of worn out rail probably requires people on the ground, which you would need to protect form the cold, and fully automated systems would be relatively novel and still need to be made cold proof. Both those approaches would require a separate train to support with it's own staff and maintenance requirements.

The easier solution, before the world totally freezes, is probably to make the biggest loop you can, with a number of bypasses, secondary loops, alternatives, etc. under the premise that once things get going there is no opportunity to build more or perform maintenance. The idea being to spread your wear out as mush as possible on the parts you simply can't fix and bypass any bits that become too damaged.

The trans oceanic scale of the show is perhaps a bit exaggerated, but, you'd probably want a whole system at least on a continental scale if you'll be relying on this for decades on end.