Transport Tycoon Forums

Is there mobile source code available?
How to build it for android?

Android support is not part of the master version of OpenTTD. I don't know who develops or publishes the Android version, but since the code is under GPL license, they are obligated to give you a full copy of their modified source code on request, so I suggest you find a way to contact the publishers of it.

It's available on github here: https://github.com/pelya/openttd-android