I swear my 6 year old grandson's favorite anything is something that hasn't been on the tracks more than 3 minutes . It doesn't help that he's basically surrounded by locos in the train room. Unfortunately Ben lives in San Jose and I live near Pittsburgh . But when he does visit, he keeps me very busy swappin' out stuff every few minutes! I will say the one car that he will ask for is a work caboose that has a welder with a flickering bright blue lighted cutting torch flame which seems to fascinate the little guy. It does look very realistic.
Rich