Hi Anthony,
Other railroads also had excursion cars; the Maine State Archives, holders of the Portland Company records and drawings, has a Portland Company drawing for an excursion car design for the W&Q/WW&F. The design for a covered structure, with partial walls and affixed to a flatcar using the stake pockets, limits one to very few options. If you look at Cass Scenic Railroad, as well as other tourist lines, they either built a roof and partial wall system to mount on flatcars OR they cut out the upper half of the side and end walls from boxcars (C&TS, for example).
David Crow