Is it? I haven’t been in one recently, but I would get people out on one side, and in on the other (which typically isn’t possible in existing subway tunnels)

Rollercoasters have a few other advantages:

- exits at every row of seats.

- less crowded (no standing passengers, never three people on a two seat bench)

- at every stop, everybody gets out, and everybody gets in.

- passengers typically are relatively fit (no small kids, no elderly)

- no luggage.

That simplifies the process quite a bit.

Of course, but rollercoasters also frequently have much more money to spend on security features, so technology first seen there shows up a while later in public transit.

E.g., modern signalling that replaces the old block signalling, or the station door systems to let people in/out which now are also in major subways around the world.