Then you need to support the train-specific OS, and find developers who know how to write code for this train-specific OS, and IT staff who can install the trai

Then you need to support the train-specific OS, and find developers who know how to write code for this train-specific OS, and IT staff who can install the train-specific updates onto the train-specific OS. As for why they would not use Linux? Leaving aside that these probably predate the wide adoption of Linux, Windows is often used in embedded devices which need graphical interfaces/displays. Again, ease of finding developers and third party tooling is a big driver.