Unusual steam engine movement

[joe935]

Steam engine is a Northern 4-8-4 with Vanderbuilt tender. Seems this problem just started recently:
When the engine is moving on a curved section of track the rear of the engine fishtails. That is it moves from side to side and the motion is very noticeable.
Any ideas on what could be causing this fishtailing motion?
Thanks

[Jim Banner]

Maybe not as unusual as you think.  Our model 4-8-4's are more closely related to real world 0-8-0's than Northerns because unlike real world Northerns, the leading and trailing trucks do not help guide the locomotive - they just go along for the ride.  Real world switchers, including 0-8-0's, have restrictions on their speed because this hunting motion can throw them off the tracks at too high speeds.

What causes this hunting is the fact that the wheels do not run perfectly straight along the tracks.  Minor irregularities can start the locomotive moving slightly one way, then the tracks over straighten it, causing it to move the other way.  This would be minimized if the wheel and rail profiles remained perfect (the same taper that allows trains to go around corners without having to slide wheels to make up the difference between inside and outside rail radii also helps keep the locomotive and other cars centred on straight track.)  But wheels and rails wear, and with time, tend to lose their taper.  So the locomotive hunts more.

With out models, it is unlikely that the wheels or rails wear significantly, except perhaps traction tires.  But dirt build up on the wheels can have the same effect.  So the first thing to do is make sure the traction tires, if any, are in good condition and that the wheels are clean.  If there is still a problem, then a tiny amount of plastic compatible conductive oil on each rail may help.  If you use one tiny drop on each rail and let the train wheels spread it around the tracks, it can do wonders.

[joe935]

Thanks for the reply Jim. But that was not my problem.
  Today i believe i found the problem. The driving rods linking the wheels were out of sync. 
Adjusted them by placing the links in the same location on both sides and the train is running perfectly now, no more shimmy.

[taz-of-boyds]

Interesting, thanks for letting us know how it worked out!
Charles