Is it possible to buy a travel curtain for hostels/trains?

You could most definitely build one yourself by using shower curtain rods, of the type that have suction cups at their extremities, and a regular curtain. These rods are usually built using two concentric cylinders of different diameters, so that the narrower one slides inside the wider one. The usually look like this (image courtesy of Bricorama):

The rod attaches to the wall using the suction cups at its extremities, and the pressure on the cups is maintained by a spring within the two cylinders. A great schematic I found on this website shows this in action:

These bars usually come in standard sizes, of which the longest seems to be 220cm, which should accommodate most hostel beds and train couchettes. They can be made out of metal or plastic, the former being more appropriate when going through security checks, as @SpaceDog suggets. Ultimately this solution requires carrying two rods of size 120cm each (as well as the curtain, of course). Whether this is a travel solution is up for discussion.

Buy three suction cups with hooks, some nylon cord, solid cloth.

Cut the cloth into two panels, sew a tube along one edge or buy some grommets to place along one edge or get some double sided velcro (hook one side, loops the other) to sew tabs along one edge.

Thread the nylon cord through the chosen attachment method on the cloth, tie a loop at one end to slip on a suction cup hook, leave the other end untied.

Put one suction cup on each end wall and one in the middle of the roof or bunk above. Hook the looped end of the cord to one of the wall mounted suction cups, hang the exposed cord in the middle on the top suction cup hook, then gently tie the un-knotted end of the cord around the other suction cup hook.

Viola, privacy curtain, tucks into a corner of your luggage, a few ounces of extra weight, only a few bucks to make and as cool looking as you want (all sorts of cloth colors and designs ;-)