This is an interesting idea for sure. I enjoy the thought of 'recycling' old things for new purposes. Though, it is likely that many of these tracks are out of use for a reason. Either they've been abandoned because they've been replaced by more efficient routes (practical use), or the land changed ownership (expense), or a combo of the two.

It would definitely be cool to create some type of tourist situation out of old lines that run through remote country, but I don't really see how this is a game changer in the slightest. There are already companies that do this exact thing, granted they are not used with privately operated vehicles.

Which kind of brings up the really bad part about the practicality of this idea. Railroads are highly coordinated efforts. Its not like a road where you can just pull off and park when you get tired or bored. Train 'parking lots' would need to be constructed which sounds like a logistical nightmare.

All this aside, I think its a very creative idea and I say explore it further. If you can deal with these challenges then I say it's definitely feasible. Don't take my criticisms as a deterrent. Really think about these things because they are important to the success of your idea. Please post updates if you pursue this further.