Very few cities have 24/7 subway service. NYC is pretty much the only one I can think of. German cities generally have 24 hour service on the weekends, but not through the week.

A few others: Chicago, Philadelphia, Copenhagen. I am a bit biased, but I think Copenhagen has the best service in that regard. Almost any main transit route runs 24/7: the metro, the 9 'A' trunk bus routes, most of the commuter-rail 'S-train' lines, and even the international 'Øresundståg' train service to Malmö. It makes a nice difference in peace of mind, because you never have to worry about being stranded anywhere.

Stockholm have comparable service level during nights as well. Although some smaller trains don't run for a few hours during the night, even on weekends.

Philadelphia has VERY little 24 hour service. Last I checked two bus routes operate along the same path as the 'orange' and 'blue' subway lines and nothing else. The regional rail service shuts down by midnight.

Ah sorry, I misread. It looks like there is 24-hour subway service, but only on weekends, with bus replacements on Sun-Thu: http://www.septa.org/service/all-night.html

In Chicago we have 24 hour service on the most popular subway routes (the red and blue lines). Several of the busiest bus routes are 24 hours as well.

London and Tokyo are both also not 24/7 (although London announced a couple of months ago that some central lines would move to 24/7 in the next few years I think).

London Underground will also be 24 hours on Friday and Saturday nights only, again due to requirement for maintenance timeslots. More info http://www.tfl.gov.uk/campaign/tube-improvements/the-future-...

Also note that "night service" on NYC subway can sometimes be every 20 or 30 minutes and run alternating on one track

Maintenance, it's an old metro and there is no second railway like in NY.

I'm guessing the cost of staff factors into it. Some Paris Metro lines are ATO, but still operator run. I can't speak to the SNCF lines, but I imagine since they're prone to striking...

No, it's that there is no time to do maintenance unless you have extra tracks. In NY you can close the one pair of tracks and do maintenance on the other. If you've only got dual track, then you can't do maintenance.