Why dig such huge tunnels just for power cables? If you need them to be accessible to people and machines, why nut just make them a little bigger and run the subway (tube or whatever) through them? I mean, you're going to have to dig the tunnel anyway why not use them for as much as you can?,(reply>), @Eric Taylor Well, now they have to be even bigger. Why not have two narrow tunnels specialised for what each one is made to carry, instead of one double-wide tunnel for both?,(reply>), @Rowan Evans<br>I can't see why there would be interference, assuming things were designed in cleaver ways.<br>    Just give the people working on the lines enough space to stay clear of the trains.<br>     As for health and safety, so long as things are well covered (something you need to do anyway) there should never be any issues.,(reply>), If you did that, problems with the power line would disrupt public transport and vice versa. And that's assuming there aren't any health and safety regulations making it impossible to put mains high-voltage lines and passenger trains in the same tunnel. And assuming public transport could make use of lines following routes from power stations to transformers.