Well, almost anything will pull better with more weight, that's not much of a surprise. 
My guess is the front truck is spinning while the rear truck sits there.
A lot of the traction typically comes from the back wheels, which is why I emphasize them. Sadly, they put the tach on the front truck, a mistake IMO. FWIW, Mine had a similar issue, I had very poor pulling power before I tinkered with the rear truck and got everything aligned and working properly.
Another thing to consider, you can always add weight, that almost always helps traction, and the Aerotrain locomotive isn't all that heavy. If you have verified the wheels are in firm contact with the rails and there's no issues with the power trucks, that's my next step.
This was my solution to a weight issue with the Lionel HHP-8, adding a bit over a pound of weight made this into a totally different engine, night and day! It used to have issues pulling four 18" passenger cars, now it pulls the whole nine cars without any issues.
