The Tunnels card does not require you to lay rails on a particular type of terrain to activate it, but just that you lay rails with it.
Yeps, got that bit
...allows for the discounted cost of laying rails on its own and all future appropriate lay rails actions (ie. any type of lay rails card performed on mountain spaces) that occur on the same turn.
It would be kind of useless if it only allowed future lay rails cards that turn to receive the discount.
Thanks for you reply! That was my thinking too, but wasn't 100% sure for the reasons I mentioned in my O.P.