Another thing I consider funny: all former Dutch colonies drive LHS. Napoleonic influence never got there it seems.
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!" ;) )
As to why: having this uniformity is useful in placing, eg, signals (which face in one direction).
Much more confusing in railways is which track is the “up” line and which is the “down” line.
I wonder how true that is, though. The metro is sort of a glorified tramway, and trams have to operate the same way round as street traffic.
The whole "which side it drives on" is moot for trains. There exist regular railways with single-track sections that trains use in different directions. I've seen terminal metro stations with two platforms where only one is used, the train arrives and then goes back into the tunnel the way it came. Many metro systems have separate tunnels for each of the two tracks connecting two stations so this notion of driving on a particular side isn't applicable to them at all.