Hi Trevor; When 15" Crown #1 was at Wildlife Prairie Park near Peoria, IL their heaviest traffic was people getting ready to go home after walking around the sights in the park. This return trip was all uphill but the worst part was about 1900'of 4% grade on a long S curve.They just beat the stack off that little engine tryingto get those heavy trains up that grade.
To help on the hill the owners installed a 12 volt motor with a sprocket and chain arrangement to the tender truck axle. Something like this may be what you saw with the Briggs and Stratton
between the frames.
The Wildlife Prairie Railroad was covered in an article in the May '82 Live Steam, if you have those in your library. Jim