It probably helps that it's just before season opening, and they are all in the workshops getting the trains ready for next month, but I think any time of the year would be great to see the amazing restoration projects they have underway.
The tour guide, Leroy, is a treasure. Do try to catch one of his shop tours if you are around Jamestown on a Tuesday. If you're a mechanic, tinkerer, or train nut, you MUST take this awesome tour.
The tour started with a pretty lengthy presentation of the history of the trains and the maintenance and restoration projects. Then we went to the roundhouse to see the engines (and some other cars, but let's face it, it's all about the engines), where we got such great detail and background on the work being done, and the workshop required to do it (ever think about a train having tires? and how to change them? And the lifespan of those "tires?" Apparently it's about 200,000 miles) The tour took us through all the workshops and was filled with neat details and answers to any question-- it's clear that Leroy knows and loves everything about these trains and the work going on here.
Shop tours are on Tuesdays at 12pm (2 hour tour), except sometimes when it's super-hot, according to the gentleman working in the store. So, come on a Tuesday in the spring to get the best tour, Saturdays, starting in April, to ride steam, and Sundays, starting in April to ride diesel. (hint-- you want steam. It takes them almost 4 hours to get the steam engines running, which is why they don't run them other days)