Can the original Ticket to Ride (US) rules be applied to the Europe map?
I would suggest that Ticket To Ride is fairly self-balancing.

What this means, applied to your question, is that the European map and destination tickets won't be as intrinsically balanced as if you'd played with the full rules.  Obviously routes involving lots of ferries and tunnels have suddenly become a lot easier to complete, and as such more attractive to go for.

Equally, though, we can assume all players know this to be the case.  If the game gains an element of competitive jostling to be first to obtain what appear to be underpriced routes, then that's a whole new layer to the game, and one which I imagine would be a lot of fun.

I wouldn't want to play Ticket to Ride: Europe with the US rules a lot of the time, but as a break from the norm there is no reason why it shouldn't be a great game.  Having to re-evaluate a "known" gameboard in the light of a dramatic change of the ruleset is an excellent strategic exercise in and of itself.