The monorail is part of TRS2006, but will also run in TRS2004. I've heard there are Level of Detail (LOD) problems in Trainz Classics, and the project makes extensive use of LOD models, so I don't think it will run well there.
The only place to obtain working content is from the Download Station. The TRS2001 CD that contains the monorail contains a number of bugs that have been fixed on the DLS.
JFE.