The whistle relay on one of my postwar tenders was sticking closed causing continuous whistle operation, once activated. I took the whistle relay out and it seemed to have a slight amount of residual magnetism. I tried adjusting the armature, as recommended in the Greenberg Repair Manual, but it still remained in the closed position once it was activated. I then put a single layer of Scotch tape on top of the armature between the armature and magnetic coil, but it still stayed closed. I added a second layer of tape and it now works properly. The tape kept the armature a small distance away from the magnetic coil so that the small amount of residual magnetism was not sufficient to hold the relay closed. It is an easy fix if a whistle relay has this problem and cannot be fixed with the armature adjustment.
If the sticking returns, go over the relay with a VHS tape eraser. I have done this with several relays and reversing units, and it eliminates the sticking.
I believe a VHS tape eraser is just a coil with an AC current flow. If so, the whistle relay should be de-magnetizing the armature anytime the whistle is ‘off’. Is that not the case?
I have made that argument many times to those who think that their e-units are magnetized. However, the whistle relay is a little different. The copper rings at the bottom of the coil shield that end from AC magnetic fields, leaving only the DC component from the whistle signal, which is why the relay operates only when that DC component is present.
In any case, putting a little scrap of tape in there is a lot easier and more permanent than demagnetizing the relay.