I'm not sure just what your are asking but for me it's the whole thing. Routes, rolling stock and locos. I espically like Prowlers locos the small ones (c16 ) the shays and even the big ones the K-27s and K-36s.
I'm currently building my own moutain type route that will be a little over 30 miles from the coal mine to power plant.
But after thinking it over, watching prowlers (C16) locos running on any route is what draws me to narrow gauge (3ft) railroading.
I'm just asking what are your favorites of each class I listed. For instance, my favorite real narrow gauge railroad is the Durango & Silverton, my favorite real engine is the K-28, and my favorite ng equipment is the chama coaling tower. When it comes to Trainz, the railroad is Northbay County, the engine is Prowler's K-28, and the equipment is the narrow gauge mining stuff like mini-orebins and such.
Prototype: I never have like prototype railroading (I come from a model railroading bqckground) so the answer is "none" for all the items in this catagory
Trainz:
Route: the one I'm building
Engine: Prowler's C16 (with a shay as a close second)
Equipment: Blue Sky Mining