> It's something I haven't really thought much about before. The simple answer is to have redundant equipment (tunnels, tracks, cars), but few would stomach the

> It's something I haven't really thought much about before. The simple answer is to have redundant equipment (tunnels, tracks, cars), but few would stomach the cost. Given that your premise is that everyone is on transit, presumably some of the incredibly gargantuan subsidies that are handed to private vehicle drivers would be used. I don't know if it's the just-world fallacy or what, but people always seem to have trouble grasping truly how inefficient car travel is. In a situation where gov't doesn't heavily subsidize them (and the drivers aren't paying the high costs associated with cars), I can't imagine that the cost of redundancy equipment required for no-downtime upgrades would be un-stomachable. On the other hand, the other thing I know is that the electorate is far too stupid and far too selfish to grasp simple concepts like this: my own city has to fight tooth and nail against car drivers as a political bloc in order to claw back a fraction of the outsized privileged position they have in the transit infrastructure. So you may be right that it won't be politically feasible despite being less expensive than the alternative.