I was thinking about it in the shower, and was on the verge of posting that maybe there is no answer and the man was trying to keep the children quiet on the train!
Any thoughts on using the addition sign only once anyone? (Using only a tiny bit of lateral thinking combined with a lot of cheating, you could spin the + sign through 45 degrees to give 13x4=52, although I dare say the puzzle would have been worded differently if that's right!)