Hey, long story short - I do not find "expanding my box over it" a compelling way to get resources, so I added fortified trains, but nuclear biters still can expload rails even if "immortal rails" setting is on

I think I know what's going on.
In recent versions of Rampant Fixed for Factorio 2.1, Nuclear Biters create special tiles that can't be built on without concrete.
I will allow rails to be built on these tiles and the rails will not be deleted

I can't give you a date for the new release yet, but don't expect it in the next few days

This also affects elevated rails?

Maybe. It's good that you mentioned them. I'll take a look.