We took our train crazed son to this museum and he loved it!
The first floor has about a dozen or more engines and specialty cars to look at. Upstairs there are displays of toy trains. We spent about 1 hour and a half looking around and climbing through train cars.
We also road the steam engine that runs weekends this year from April to September. The ride costs extra, and is about 40 minutes long.

The museum is right next door to the Old Sacramento Town tourist area, with restaurants and knick-knack shops, and it's located on the banks of the Sac River. We brought our own picnic lunch, and being a State Park facility they had picnic tables that were pretty clean.

The only price shock for the whole day was the parking. We were there for about 4 hours and the parking came out to $10. I suppose we could have
bought something and validated at one of the shops (the museum itself doesn't validate), or there is some metered street parking. But, all-in-all, I think it's more convenient to just pay and park in the garage.

Oh and one final tip, discretely avoid the museum souveneir shop if you have a kid. If you're like us you have plenty of train paraphernalia already, and you don't need more. There was plenty of whining and arguments going on between parents and kids when we were there.