The California railroad museum is definitely one of the best in the country but if you want to see potentially the largest collection with lots of cosmetically and mechanically restored equipment, you have to check out the Illinois Railway Museum

Of course if you're more into narrow guage the Colorado Railroad Museum is hard to beat. Or if you just want an epic train ride, take your pick of the Durango and Silverton, the Cumbres and Toltec or the Royal Gorge Route

Personally I've been to the California Railroad Museum (back when they were still Orange Empire), the Illinois Railway Museum, the Cumbres and Toltec and the Colorado Railroad Museum. The IRM is great anytime but especially if you come during an event weekend like Labor Day or Memorial Day because they run their mainline and trolley loop at full capacity during the holiday weekends with as many as 8 trains running at once, but they also have enough accessible equipment even on weekdays when they just run a single electric interurban to make it still worth a visit. The Colorado Railroad museum whelmed me when I was there on a weekday, but I'm sure it's far more exciting on a weekend or event day with more going on. It's pretty small but has pretty unique collection (including 3! of the galloping gooses) Cumbres and Toltec was definitely a worthy bucket list ride, and when I was at the California Railroad Museum I joined a tour group, had the entire group split off, so the guide took me off the beaten path and gave me a really in depth tour of literally everything he has keys to and shared a ton of neat information about a lot of the equipment (such as the interurbans that were specifically built to serve one of the college campuses. As he put it "y'know how in Wisconsin bored college kids will go cow tipping? Well here they'd go street car tipping, so they built these to be much heavier so they couldn't be tipped and ran them exclusively at the campus")