[skissane]

You have a legacy safety-critical system, which incorporates legacy hardware peripherals. How sensitive is it to changes in timing? You may not actually know. Do you want to do the engineering analysis necessary to prove that replacing one part of that system with potentially different timing is not going to cause problems? Or do you just seek out a replacement whose timing is as close as possible to the original?

The big issue may not be with the trains themselves but the communications protocols used to talk to signalling equipment and other peripherals. Changing the timing in the communication with them may lead to problems.

And what if the original software has race condition bugs which have never been surfaced, and the occasional inaccuracy in timing starts to surface them? Good luck fixing bugs in some obscure piece of PDP-11 software that was written in the 1970s.

[mycall]

You could always setup a train in a box system and iterate all the control logic sequences to verify being within margin of error. Once you know that, equipment substitution is straightforward.