Can we extend the game by playing Multiple Boards At Once?
Firstly, to extend the game you will need to add twice the train pieces as well as 2 boards. Maybe you should double the tickets too. This soon degrades into playing two different games at the same time, where you choose which one to play each round.

The second problem is all the advantage is on the side of playing on only one board. This is because adding more trains to a single board makes it easy to get more tickets on that board, while trains on both boards leads to spreading yourself thin. My solution to this is to modify (or replace) the longest railroad card to include a benefit for using both boards.
(eg. longest railroad on board A * longest railroad on board B)

I think this variant is not a good idea, but you can try it and see what its like. Seems to me  an interesting experiment at least.