Here's one worth checking out - Roaring Camp Railroads in Felton, California.
Pick either the Redwood Forest Steam Train Ride (1.15 hrs) or the Santa Cruz Beach Train Ride (3 hrs). They also have special rides like a Starlight Evening Train Ride or a Great Robbery Train Ride.
We tried the Steam train ride, which was a lot of fun for kids and adults.
The property has a duck pond and large picnic grounds so you can easily spend half a day on site.
Once a year, they have a Thomas the Tank Engine Day - must visit for Thomas-crazed kids!