I have two 4-8-4s and a 4-6-4 on my layout.  The radius curves are 22" which the directions advise is the minimum.  The tracks are level and no matter what speed the front truck on each comes off the track at a certain portion of the curve.  If I run the engines in the opposite direction they stay on but counter clockwise they come off.  Any suggestions?

Thanks for the information.

Is there a specific formula for a smoke engine.  On one of the bottles it listed mineral oil as an ingredient.  Is that something that can be used or is it mixed with something else?