This is the Robert Dollar Lumber Company Number Three engine (recently restored). It is reportedly the last wood burning steam engine built for use in the U.S.

http://www.ncry.org/roster/s_03rd/rd3.shtml

Currently still in service as part of a "tourist train" running between Sunol and Fremont, California through the Niles Canyon. It is a fun trip running about an hour. If you are ever in the Bay area in the summer, you might consider this as a side trip. If you are interested in a visit check out Niles Canyon Railway on the web.