The museum has HO and N scale model RR's in one of the buildings. If you go, get a guided tour. The docents will share a lot of history and interesting information. There is also a dining car adjacent to the museum where you can have a sandwich for lunch. I believe it is still open. It isn't luxury dining car stuff, but it is still fun eating a sandwich and chips in a dining car.
A tour with a docent will take 2 - 3 hours.
It is still quite hot and humid this time of year in Savannah, so dress appropriately and wear sunblock.
If you want to see real trains, the safest spot is the Amtrak station which is about a 10 minute drive from downtown. The CSX main line runs through there and you will see freight and Amtrak trains.