http://edition.cnn.com/2016/08/22/europe/night-tube-riding-londons-24-hour-underground/index.html
New York has already realized 24-hour subway. It's because some lines have quadruple tracks, half of which are for trains and the other half of which is under maintenance. After a while, the former is supposed to be under maintenance, the latter is for trains.
But, the biggest question is how London underground has been able to run 24-hour service without the similar system.
