IDK. A lot of these systems are stuck not just because of complex business logic. That would be tedious to rewrite, definitely take longer than a weekend, but AI systems are actually a great use-case for helping in refactors. I think a bigger issue can be custom drivers / embedded technologies that require specific protocols / assumptions about servers. And those embedded areas may sit in hundreds of trains + train stations -- not impossible to update, but definitely more challenging to verify upgrades 100x times instead of one time. And the consequences of something bad can be train collisions = death. So it's easy to see how an organization leadership committee may decide "if it ain't broke don't fix it" because of risk with no real upside aside from a marginally lower salary cost of one maintainer -- if that.

Not even sure AI could help about it, those software might run over system where the documentation never got out from physical paper support, maybe even hand notes.

Talking Win 3.1, it means using Quick C / MFC 1.0 and co. that I image no today AI would learn as deprecated.

Visual C++ 1.52c (with MFC 2.50) is the last version to support 16-bit Windows and includes full electronic documentation (in Windows HLP and WRI format, admittedly, but it's there).