Overnight train luggage safety concerns in Europe

Mark Mayo covers a lot of good points, but when I've been worried about similar I use a bag/pack with locks. So it has two zips that lock together with a padlock, then I put a bicycle chain/lock through that and lock it to the shelf/seat -- there's almost always something. Sleep with the keys on a chain round your next if your extra paranoid. Then I sleep with a smaller bag under/beside my pillow with the really important stuff in it.

If someone's looking to steal stuff they're not going to bother with the risk of breaking the locks and getting caught -- they'll probably just look for other stuff.

And also try not to be obvious about what you're carrying -- a laptop bag or a camera bag are a big draw, but you can put those bags inside a scruffy old suitcase and it'll attract less attention.

Sadly, a lot of the time you don't have to make yourself not a target -- just be less of a target than the other people around.

But have good insurance anyway.

I would like to add some points to Mark Mayos already good answer:

Imagine you are a thief:

• You cannot escape from a train which drives with a speed of over 160 km/h (100 mph). The very best case will break both of your legs, crippling you. You cannot hide either, because all inhabitants of the cabins know each other and also know who was in the cabin when an alarm is raised.
• While you may be able to open the door silently, you have now four to six persons before you of which you know nothing. You do not know if they are deep sleepers, light sleepers or nearly awake because they need to take care of urgent business.
• You have several different breathing rhythms and persons left and right so you are unable to focus on changes which could warn you if they are waking up.
• The things to steal are either in the high cubby-hole or under the seats. You must search unknown containers in unknown positions in an inconvienient position in direct vicinity of the sleepers in nearly complete darkness. Good luck.
• Worse, you have no idea where the sleepers store their valuables and if they have valuables at all, so you have absolutely no idea for what you are taking this extremely high risk.
• If they wake up, you are hopelessly outnumbered. You may get away with the surprise effect and run, but as already told, you cannot flee.

I therefore conclude that the risk that a thief will attempt to steal sth in a sleeper train is relatively low.