Some years ago, before Android and iOS, I used to have a map for Windows phone (and web) that allowed you to download per-city packs of subway information that included schedules and connection information for cities around the world.
You were able to enter two stations, one to start and one to go to and it would plot you all possible subway connections between the two points.
I have been looking quite a lot now on the internet and the Android app store for something like this, bought even a couple and refunded them. Most of them simply are a collection of maps where you have to trace the route yourself instead of having a real "navigation" for subways.
As far as I remember the application I used to have was called "Metro". It was not open-source IIRC but the data was provided by the users. Anyone remember this? Does it or something similar exist?