Why does GPS not work in a train? GPS signal strength is Zero while I'm in a (electric) train.

Why is does my device not get a GPS fix in a train?
In some trains you sit like inside an Faraday cage, as they even metallized the windows -- thus no signal can get through. That's also why you have special "mobile phone" zones in some trains: Here they placed something like a little "repeater" inside this section of the train, capturing the signal from outside the cabin (and transmitting it back the same way, of course). However, they don't do that for GPS.