The Thomas Cook Continental Timetable, published monthly at this time, would be the most reliable source of information - if you could find a copy. A facsimilie of the August 1939 edition was reprinted in 1987 edited by JH Price and may be in libraries (or you can get a secondhand copy for a reasonable price).

It goes without saying that the outbreak of World War II would have disrupted things considerably. Bucharest was on one of the Orient Express routes, and had there been direct trains to Paris beforehand - I'm not sure if there were or you needed to change - there wouldn't have been now. But the August 1939 guide should give you a good idea of what was possible between Romania and non-belligerent nations at that date.

It might be worth having a look at some of the later Bradshaws guidebooks if you can. You'd be looking more at the First World War period over the Second but could still be of use as a wider frame of reference for early 20th century rail travel.

Were railway timetables posted in newspapers at the time? If so, you could try's Google's searchable news archive.