The subway of Paris usually has only two tracks. They have started making it automatic (and so they need to use a modern signalling system) and they seem to be

The subway of Paris usually has only two tracks. They have started making it automatic (and so they need to use a modern signalling system) and they seem to be successful at doing it without disrupting traffic too much: http://cometandnova.org/improving-service-through-automation-in-paris/ http://cometandnova.org/improving-service-through-automation... and https://en.wikipedia.org/wiki/Paris_M%C3%A9tro_Line_1#Automation https://en.wikipedia.org/wiki/Paris_M%C3%A9tro_Line_1#Automa...