Replacing the whistle rectifier resistor wires on a zw transformer with modern resistors

20-30 watts should be plenty. Lionel's rheostats were only 20 watts.

You can eliminate the resistor altogether if you replace the rectifier disk with a Zener (NOT SILICON) stud mounted diode. 

I have already replaced the 1041-13 rectifier disc with 40 amp 1200 volt diodes. I don't know if they are zener or not. I will attempt to find out. 

   I have never used a zener diode on my own external whistle buttons. I'm not saying there aren't advantages, but nothing is cooked here with 8-10a general purpose diodes inline.

 The suggested diode is likely fine if you got the number from someone else here.  If it dies THEN maybe worry about zener. Until then, it's a curiosity imo.

I do have two pw that wont respond to diodes and two modern locos that dont respond to pw discs, with boost nor without.  I stopped fighting them and reach for the button that works

I've determined my diodes are not zener. I am interested in the number or specifications of the appropriate zener diode for the lionel zw 275  and kw 190 watt transformers. Thanks for all the replies. Paul