Actually, in the French Eastern areas that used to be part of Germany trains drive on the right and so I believe that regular trains between, say, Paris and Str

Actually, in the French Eastern areas that used to be part of Germany trains drive on the right and so I believe that regular trains between, say, Paris and Strasbourg change side on the way, but TGVs stay on the left. Now, as to why French train drive on the left, it seems that this is simply because French railways started in the early 19th century by copying what the British were doing (although according to Google the SNCF's official position is "absolutely not!" ;) )