My 3 year old daughter loves trains, so we stopped in to check it out. Cost ~ $12 for our admission, which I thought was reasonable. She had a blast climbing in and out of the caboose, looking at the models (the Thomas the Train model was a big hit!). I enjoyed looking at the old artifacts and learning a bit about the B&O history (I could've gotten more out of it but I was busy chasing her around). It just so happened that a real train rode past while we were there, which made it all the more special, until my daughter got fixated on wanting to ride the actual train - it was getting close to lunch and naptime anyway so we left before she really melted down. The inside was bigger than I expected, but it was still small.
All in all, a fun time - glad I checked it out, and I could see bringing out-of-towners or friends with kids to check it out.