[St. Lou Stu]

Like others have said.... Polish the axles/nails and remove the burrs from the nail head. also.... machine down the head of the nail to the smallest diameter possible while it still holds the wheel on.
Weight should be about 1/4" forward of the center of gravity of the vehicle without weight installed. Max weight allowed is a MUST!
Also, install the axles at a slight upward angle into the body so that when the wheels are installed only the outer edge of the wheels touch the track. sand and smooth the wheels to a near mirror finish too.
I may be forgetting some.... but the interwebs are your friend.

Most important of all..... let Jr. do most of the work while you teach him why it is you are doing each of the above.