@redford and @mrtick held an unrecorded talk a bout this at OhMyHack in Warsaw - I unfortunately couldn't make it because of Munich snow.
For now this is making the rounds in Polish-speaking sources, but we do have a talk scheduled about this at 37C3, in which we plan to do a deep dive into this and actually publish our findings.
@zaufanatrzeciastrona 's article about this: https://zaufanatrzeciastrona.pl/post/o-trzech-takich-co-zhakowali-prawdziwy-pociag-a-nawet-30-pociagow/
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.
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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.
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q3k 
Michał Kowalczyk
