They could make their own parts. They just need to redesign out any components that can't be sourced, manufacture the parts and run a training and competency regime to retain enough skilled people to maintain, diagnose, repair and renew the system (this can be very labour intensive for electromechanical interlockings), all to safety critical standards. Since they are amongst the last people using some of that kit, little or none of that cost is amortised over other users and all the risks sit with you.
You're right that obsolescence tends to come around faster on modern systems. However, with a manufacturer supported product they are managing the obsolete components for you. If they are smart the safety critical stuff is properly encrypted to go over the network so you use off the shelf the kit for things like wireless comms that can be swapped out relatively easily. All the safety critical stuff will be in things like vital processors which will need more careful management but should last longer. The cost of all this is amortised over a number of organisations.
