PTM, overnight travel on any train is a time to take extra special care of your belongings. Fortunately the Ukrainians have security pretty well sorted. Each carriage has an attendant. The compartments (other than the dormitory style) have door locks, plus a special latch which means it can only be opened a few centimeters and this can't be accessed from outside. Some compartments have lockable metal boxes below the bottom bunk for storing valuables. Make sure you use ALL these safety features. maybe take along an extra couple of padlocks with your own unique key just to be sure.
Phil from the safety hub. about 7 years ago