10+ years ago there were very few checks — the police would need a specific suspicion to carry out a check. I saw this once on a daytime train, when they checke

10+ years ago there were very few checks — the police would need a specific suspicion to carry out a check. I saw this once on a daytime train, when they checked three people who clearly matched a description, and arrested the third one! I've also seen this twice when disembarking a within-Schengen plane, with police on the jetbridge checking documents — until they find the person they want. The neverending "refugee crisis" (the official reason several countries have introduced checks within the Schengen area) has allowed more frequent checks, but it seems fairly random whether or not they happen. Out of perhaps 10 overnight journeys, I've not been checked going south or east, but around half the time I've been checked going into Denmark from Germany. It's so disruptive that I no longer take the overnight train into Denmark. You should prepare for it — put the passport in the mesh holder so you can just hand it over — but I find it difficult enough to sleep on a moving train anyway.