I agree with some of the posts above that, having eliminated the transformers as the source, its probably the brushes arcing, especially as they make/break two adjacent armature contact plates. More load on the engine increases the current the motor draws, thus increasing the strength of the signal emitted from the arcing.
So one attempt to isolate the problem is the motor would be to remove the body of an engine, use tinfoil to encase the motor, making sure to connect the tinfoil to track ground. This would create a shield, that would lower the emissions getting from the motor to you.
My little knowledge of motors (that can quickly get me into trouble) is that if the source is the brushes arcing, using DC or switching to DC motors (which today in our trains still have brushes) may not help very much. Now, as I have seen wished for in other posts in other topics, if DC brushless motors were available for our trains, the would have the best chance of getting rid of the emitted noise.
