S and Sn3
To me, S Scale is the ideal scale to work in. It's bigger than HO, which makes the models easier to build (I'm a klutz and need all the help I can get). Also, the details show up better than in HO. It's smaller than O Scale, so you can fit a lot more railroad into the same space. This page (scroll down about 1/3 of the page) has two pictures showing the same scene with S and O Scale equipment that shows what I mean. By stroke of luck, S Scale also has the best equipment availability for modeling the Southern Pacific Narrow Gauge.
Locomotives
The situation for Souther Pacific locomotives in S scale is much better than in HO, at least in Narrow Gauge. Railmaster has kits for Southern Pacific engines 8, 9, and 18, the "Little Giant" diesel, a T-12 that could be built into SP 12 and 13, and a Consolidation that could be built into the second SP #1, They also have Alco RS1/2/3 and GE NW2/SW7 engines in standard gauge. River Raisin Models is importing 6 versions of SP's MK-5 and MK-6 2-8-2s next year and 3 SP 2-10-2s.
Rolling Stock
P-B-L makes 3 different versions of SP narrow gauge box cars, and sometime in 2008 will be importing brass versions of the 400, 401, and 467 combines. Plans in 3/16" (S) scale for many of the SP narrow gauge cars are available for purchase at the Laws Railrod Museum. Since S is easier for me to work with than HO, scratch building wood rolling stock shouldn't be a problem for me. Standard gauge S Scale rolling stock is a bit more of a problem. The cars are steel, and I haven't yet found a commercial source of good, accurate S Scale frieght cars. Scratch building a couple of C-30-1's shouldn't be a problem.
Structures
Will be a mix of kits and scratch built. Since no SP structure kits are available (at least that I can find), any SP structures will have to be scratch built. I heard a rumor that P-B-L will have a kit of the Laws depot and turntable sometime in the future.
Updated November 26, 2007