This situation arose last time when we played 1825 Unit 2 with my spouse :
At the beginning, the LNWR laid tiles in order to reach the GCR base tile (still empty). Then further in the game (phase 2) a green tile is laid in Sheffield / Rotherham and the LNWR place a second base on this tile.
When the GCR float, his initial base is placed and by the way the "network" of the LNWR is split in two because I can’t reach any more my second base in Sheffield / Rotherham from the LNWR initial base.
My question is : May I run a train from my base in Sheffield / Rotherham to an other station (of course not beyond GCR base) even if the base is not linked any more to my initial base ?