Regarding driving on the left:
- if you change over, most people get used to it pretty quickly. I read that most crashes involving British drivers in France happen within a few miles of the ferry landing. I live 2 miles from the Channel Tunnel Terminal so cross over quite frequently, motorcycle or car. No problems.
- Why drive left or right? Apparently it's quite true that in the Middle Ages coaches and wagons went on the left, because they followed the armed horsemen who did it that way so that their right arm (sword arm) was on the correct side for battle if an enemy came the other way. Likewise it was correct for making a raised right hand well visible to an oncomer if you wanted to show you were not holding a weapon.
- In the French Revolution (1789) everything changed, to mark the difference with the old system. Some of the changes turned out to be permanent, like changing to right-side road use. Soon after, Napoleon temporarily conquered most of Europe and took that change with him - and it stayed. But he did not conquer Britain so we stayed on the left! Neither did he conquer Sweden and they kept to the left until about 1955. The French change would have been similar to that in the US - to mark the end of domination, whether by kings or by the Brits!
As for locomotives, the driver (engineer) sits on left or right mostly depending on whether in his country trains on double-track run on left or right. Left in Britain (of course!) and in many countries including France where the first railways were built by the British; right in the US, Germany and others. These days there is always good visibility of railway signals - it was not like that when they had steam locomotives - then the fireman had to look out as well.