The P&SR was an electric interurban, incorporated in 1903, with a main line between Petaluma and Santa Rosa and branches from Liberty to Two Rock and from Sebastopol to Forestville. The riverboats carried produce and freight transferred from the trains at Petaluma--the freight cars themselves were not carried on the boat. The line became a subsidiary of the NWP on February 26, 1932, and passenger service ended on June 30, 1932. Electric freight service continued until dieselized in 1947 with GE 44-tonners. I have never heard of steam being used on the P&SR, except perhaps during track construction (although the NWP did have a separate, parallel branch from Santa Rosa to Sebastopol, the last of which was abandoned in 1947). Starting about 1966, SP units were used--for years, it was SW8's 1116 and 1128, later SW1500's 2592 and 2595 (although Alco S-3 1025 spent most of 1969 there). The last freight operation occured in 1983 and the line was formerly abandoned June 18, 1984. The only remaining trackage is about one mile along the Petaluma waterfront, long embargoed due to bridge deterioration. As for preserved equipment, interurban car 63 is restored and operating at Rio Vista Jct., box motor 8 (body only restored) is owned by a private party near Sebastopol, and caboose 1 is undergoing restoration by the NWP Historical Society in Petaluma. I don't believe there is any P&SR equipment at OERM.
Suggested references are Fred Stindt's "Northwestern Pacific Railroad Vol. 2" (where I got many of the above dates), and there was a "P&SR in the 1970s" article in the NWP Society's "The Northwesterner" a few years ago. There is a group actively seeking to restore trolley service to downtown Petaluma--their website is www.petalumatrolley.org, which includes their plans as well as history and photos.