Measuring the speed of the subway

I have often wondered how fast the subway is going. Without knowledge of the distance between lights in the tunnel, it seemed impossible to determine without an accelerometer. But here is a way:

Two people with synchronized watches at opposite ends of a subway car note the time when passing certain landmarks, for example lights in the tunnel. Measure within the car the distance between the observers, and divide that by the time difference. This gives the instantaneous velocity.

Two video cameras with synchronized timestamps also work.