My understanding is that the model T powered speeders as used by West Side and Sumpter Valley are "Casey Jones" Models offered by Northwestern for narrow guage use.
I went to a great deal of reserach and dimension guessing to make an HO model of the Sumpter Valley unit only to find later that Summer, that one of the Westside units at Fish Camp is virtually identical.
John Taubanek then informed me that they are the Casey Jones models.
I wouldn't mind having one of them myself.
Lon Wall
Trainmaster Sumpter valley Railway