24
It's rather easy actually
You look at the distance between the 2 tramstops and look how long it take for one tram to follow up on the other tram.
Then you have the window (for example tram 2 follows tram 1 up with 7.5 minutes), if you know what a tram averages on 25-30KM/H and tramstops are on average 700m apart then you know what the tram takes around a minute/minute and a half to bridge this distance. Do this untill one end of the tramline and then you have the timetable from that stop untill the terminus, then repeat it to get the way back.
I know, rubishly explained but that's how I think