Some of our engines jerk and stall on all but pristine tracks, while others seem un-affected by the dirt. Only a tiny fraction of an inch is on the track for any one wheel. Even on the prototype, a mile long train will have contact on the rails measured in only a couple square feet at most. The model railroad has proportionally smaller amount of contact area, that is where the electrical pickup comes from. Normally when the wheels run onto dirt, and insulates the wheels from the source of electricity, the other wheels will have contact to metal and will make up for the electrical loss.
Dirty conditions can lift several wheels off the rails simultaneously, causing the engine to stall.
In engine devices like flywheels, cause the momentum of the spinning weight keep the motor spinning and push the engine foreword, hopefully past the patch of dirt onto cleaner track. In an effort to improve our electrical pickup, we are now connecting our many engines together electrically. Wires are run from the motors of each engine, to even out the electrical pickup. As a pair of wires cross near the couplers, we refer to this as MU cables. By connecting several engines together we increase the chance that at least one wheel of each polarity will be in contact with the rails.