I don't know what are express subway stops in NYC and if they have a higher top speed than regular subway cars, but I doubt it unless they have separate tracks

I don't know what are express subway stops in NYC and if they have a higher top speed than regular subway cars, but I doubt it unless they have separate tracks - they might be faster by skipping stations but that's unrelated to the topic. I'm not saying it doesn't exist anywhere outside Paris anyway, just that it already exists outside Beijing. I'd expect London to have a similar system, too. https://link.springer.com/article/10.1007/s40864-015-0012-y https://link.springer.com/article/10.1007/s40864-015-0012-y says about 1.5 m/s² is the maximum acceptable acceleration rate for passengers. My local metro in Lille uses 1.3 m/s² and I find it barely acceptable. But that is only acceptable anyway over a short time, and for a trip under continuous acceleration/deceleration it should stay under at most 0.5 m/s². I'll let you do the math for how many seconds you can gain on a 500 m stretch between metro stations. Also, RER or TGV lines are really just as smooth as the maglev I have used in Incheon. Rail is very smooth and quiet on modern continuous welded rail. On TGVs on dedicated lines you usually don't notice when the train starts.