I'll echo Kevin's comments and elaborate a little:
- California State Railroad Museum is a MUST visit. To me the train ride is OK (little to no grade, with few opportunities to see the locomotive work), but their displays are second to none. Definitely be on your guard in Old Sacramento though.
- Railtown 1897 runs trains Wednesdays, Saturdays & Sundays. For a while they were running steam on Saturdays only - not sure if that's still the case. Either way, it's a sweet ride, with steep grades and lots of stack talk on the return to Jamestown. The roundhouse and yard complex also one of those "frozen-in-time" places - highly recommend.
- Western Railway Museum "only" runs Electric cars - however it travels through some pristine rolling hills (protected mostly by the large Wind Farm in the area), and is an enjoyable ride.
- Niles Canyon Railway is in a bit of a steam "hiatus" at the moment - we test ran Clover Valley #4 a couple weeks ago, but it's not officially on the schedule yet. Either way the train ride is well worth it (admittedly with a little bias). Drop me a PM if you'd like to tour the yard as well (lots of interesting projects, between the Krauss-Maffei locomotive, GN Ranch Car, and SP 1744; the Skookum is still lurking around as well)
- Justin