Just as an aside to my original post I have just purchased a Southern Railways USA tank from Model Rail Magazine. These engines are made by Bachmann, and it clearly states in the instructions that a Relco or simiar high frequency track cleaner should not be used with them. Whether this applies to other Bachman locos I don’t know – maybe it is a different type of motor used in this particular class of loco?
Also I purchased a S/hand Portescap RG4/1219 at my local show; the original instruction sheet that came with it states that they require a smooth, pure 12v DC power supply. Surely a high frequency track cleaner is going to disrupt any semblance of a smooth, pure 12v DC power supply?