How would that make them not interoperable? A metro system is just an underground electric railway. The only serious difference is how the power is delivered to

How would that make them not interoperable? A metro system is just an underground electric railway. The only serious difference is how the power is delivered to the trains, it's usually some sort of third rail instead of an overhead wire. 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.