I can finally reveal some research I've been involved with over the past year or so.

We (@redford, @mrtick and I) have reverse engineered the PLC code of NEWAG Impuls EMUs. These trains were locking up for arbitrary reasons after being serviced at third-party workshops. The manufacturer argued that this was because of malpractice by these workshops, and that they should be serviced by them instead of third parties.

1/4

The key unlock was deleted in newer PLC software versions, but the lock logic remained.

After a certain update by NEWAG, the cabin controls would also display scary messages about copyright violations if the HMI detected a subset of conditions that should've engaged the lock but the train was still operational.

The trains also had a GSM telemetry unit that was broadcasting lock conditions, and in some cases appeared to be able to lock the train remotely.

3/4

We found that the PLC code actually contained logic that would lock up the train with bogus error codes after some date, or if the train wasn't running for a given time. One version of the controller actually contained GPS coordinates to contain the behaviour to third party workshops.

It was also possible to unlock the trains by pressing a key combination in the cabin controls. None of this was documented.

2/4