Thanks! Turns out the MTA does keep a pretty detailed schedule (not that they manage to stick to it much, these days) in the GTFS format, so it's not hard to compute travel times client-side. As a plus, GTFS seems pretty-widely used, meaning it shouldn't be so hard to port this to other transit systems!

Would this method work for GTFS Bus routes?

I'm also keen to port it to Sydney's train network as we have GTFS and GTFS realtime!

GTFS realtime is a lot of fun, I built a simple JSON real time API for Sydney buses which could be easily ported to other GTFS realtime feeds: https://github.com/jakecoppinger/sydney-bus-departures

It should! As far as I know, GTFS is pretty transport-mode-agnostic. (I imagine the bus stops are a bit more dense, though — it helps that the NYC map has a recognizable set of line colors that remain familiar when you jumble it all up)

Thanks!

huh. I didn't realize there was a standard format. I wonder if there's one for Melbourne. It took forever just to get Melb's transit data into Gmaps et. al.

IIRC, this was the main reason it took so long. Google require GTFS formatted timetables for Maps, the Vic Government released the timetables eventually but there were issues, much finger printing was done, and eventually it was rectified.

So yes, there will be GTFS formatted timetables for Melbourne, no real-time though.