The three Polish security researchers who hacked DRM-locked trains in order to fix them explained exactly how they did it at the Chaos Communication Conference in Germany over the holidays. In the talk, the hackers explained that the train’s manufacturer, NewAg, put in a series of “lockup mechanisms” that were designed to kill power to the train and break a train's compressors if certain conditions were met. An "emergency stop" signal was also tweaked in locked trains.
The trains were designed to break if they sat idle for 21 days or if a GPS detected them at independent repair centers or competitors’ rail yards. Perhaps most interestingly, one of the analyzed trains had code in it that was supposed to artificially lock the train’s compressor on the seemingly arbitrary date of December 21; a NewAg train then actually had a compressor malfunction on December 21.