Comment by delta_p_delta_x

> whether you could put a train from 1845 on a modern track

This quite literally happens all the time in the UK. There are heritage railways, and steam rolling stock regularly shows up on various routes as part of heritage rides.

On the other hand, I ride almost daily on a track bed that was laid almost 180 years ago, and through tunnels and on viaducts that are equally as old. The rails, sleepers, signals, and rolling stock have all changed, no doubt, but the base infrastructure is old hat.

The oldest bits of the London Underground are from 1863. The tunnels are exactly the same, the stations and stairways, and even some of the sign boards completely unchanged from then.

Railways are the epitome of forever infrastructure.