I am happy you liked my little Game.
To simplify it, as soon as each station is connected with two roads (or one upgraded), you reach the Endphase.
During that you can build or upgrade 4 roads. After that you will get a new board.
Two edge cases:
- If you upgrade your old road long enough you can also progress. (A old failsafe still in place)
- The 4 roads during the Endphase can in theory change into any number from 1 to 9