I'm still a little confused as to how this shows where a train actually is, it seems like this is only a solution to know when you're on a train or not. Does it just guess based on what trains should be moving at that time?
I'd assume it's the length (in time) of the vibration since the last stop. There may be edge cases where if the train is moving slowly, but overall it's a smart technique.
If there's a delay that will really confuse it though, either if your train isn't going at the speed the app expects or it stops at a point outside of a station
It probably would, but if someone has the app running, then the app knows what train they are trying to get on, and so if the data is all consistent with being on a train they are probably on that train. The app also knows where the train is, so it knows where you are as well. It won't know which train car you are on, but that normally doesn't matter.
This probably is wrong if trains for both directions stop at the same time from opposite sides of the same platform and you get on the wrong train. However this is a rare enough case that we can ignore it - but very annoying if you are that person (this case happens to most transit riders once or twice in their life). Even when this happens there often is other location data scattered around so you would likely only ride a stop or two before the app realizes you are on the wrong train and can reroute you.
