Western Railway Museum

We visited this museum during the Christmas season, which I imagine to be one of their busiest, with a fair number of families bringing children for the Santa Rides. Make sure to call before your visit, as there might be changes from what is posted on the website.
The trains range from municipal electric train to older, massive rail cars and at least one huge, black locomotive.
The place is run by volunteers who evidently put in a lot of time and care into keeping the museum entertaining and relevant, so we'll visit again at a different time of year for a second look. Being located to the side of CA-12, pulling off into the turn out lane can be a bit daunting if traffic moves along close and fast so make sure to signal well in advance. If you're making a day out of it, it's worth driving to Rio Vista for something to eat, where several restaurants and diner offer pretty good fare at good prices, certainly better than the generic fast food places you pass by Suisun City.