If you want to do so ( and not with a hard start/stop ), you need a separate H-Bridge for each of the trams. So each of the trams has is own power source which is regulated (with PWM) to control the speed of the trams. You can use relais to connect the according power source to the block(s) where the tram is running.
P.S. what kind of sensor do you plan to recognize when the tram has reached or left a block?