Are you going to be doing a lot before and after? I would recommend Bob Gurr's Waltland Bus Tour, but that goes from 10am-3pm and is kind of pricey. However, it always guarantees you access to the barn. He only does the tours on the day the barn is open (every third Sunday). However, if you just wanted to go alone, I would get there as early as you can because it tends to get VERY crowded. But, if you wanted to make the most out of the trip there and you don't mind the crowds, get there I believe between 1-2. That way, you might get a chance to meet Bob Gurr also without paying for the bus tour. It's pretty much just pick whichever one you wanted most. When the barn gets crowded you can't stop and meander. They will rush you right through it. However, if you get there early enough you will probably get to take your time. Also, don't rule out taking a ride on one of the trains. The people who built them are always delighted to have passengers.