There was a period of time when the counter for riding subways was not turned on, so... you'll have to keep riding a few more times. How many? Hard to say - it all depends on how many times you rode the subway while the counter was off.
There may also still a bug, that if you get off and then on the train too fast it doesn't count as a seperate ride. I leave the station for a sec to be sure I do not get hit with that one.
I definitely think the counters are off. and has something to do with how quick (possibly) you are at getting on/off and (possibly) when there's more then 3 of you doing it... I was +79 trips over before finally registering for the 251 badge!
I got my 251 badge at 261. Since becoming aware of the instancing problem, I began dropping a bean, bubble, grain, etc. while riding. My theory (which may be wrong!) was that if the dropped item was there when I got back on the train, then I hadn't gotten a new instance (and thus, that ticket-purchase wouldn't count). I did get back on a train once to find my bubble still there, but since I was already at about 240 when I started using this trick, I can't confirm that it's a reliable indicator of a badge-worthy trip.