Because they run on the left in Britain and France followed suit when building its railways. Many other European countries are left-hand running on their railways. There is an exception in France as outlined above.On the French thing, I thought that it was odd going to the South of France by train all TGV and the lines to Beziers all run on the left
Do all trains in France run on left. Any one know why?
In Alsace-Lorraine, when Germany invaded the region, it switched the existing railways to its own right-hand running, and instituted that for the lines it built there. When France regained the region, it didn't change it back but left it as right-hand running and this remains today, though much of the network is fully bi-directional across France anyway.