For some time I'd been tracking the development of a train simulator) called Open BVE
The engine it uses is potentially flexible to allow it's use for the development of dark-rides of the flat, tracked design.
It also available under a "quasi-open source license". (Source code here:- http://github.com/leezer3/OpenBVE)
I'd be very interested in seeing if the themed attraction designers here considered such tools useful in building mock-ups of their own ideas, in a modified version of the engine. (I've even suggested a dark-rides simulator fork of the code to the developers here - https://github.com/leezer3/OpenBVE/issues/98 )
I'll be posting a seperate post about 'fantasy' railway ideas in the near future.