I've never had any kind of failure with these designs, but keep in mind that MPC probably did not think originally that these would be asked to pull much more than their 3 car consists when drawn up. Also, these sets were designed for & came with 50 watt(input) and sometimes smaller 16 volt transformers, not KWs or ZWs.
I have found that these do run a little warmer with modern electronic power supplies as opposed to straight sine wave transformers.
If yours has heated up appreciably, the windings may have now broken down.
