“The problem with this design is that most people don’t understand state machines, and this design confused a lot of people who were unable to lock the door correctly, or believed they’d locked the door when they hadn’t.”
Hitachi’s Class 800 swapped a real lock for a lever wired to a microcontroller, then guarded it with a tiny metal pin. Hold the lever at just the right spot and the pin fires into thin air, so you can lock an empty toilet and jump out before the door shuts. One attempt confused the controller into out-of-order mode. A train toilet now needs a reboot.