So after some poking around it appears this is the factory replacement tether for PS2 3V steam locomotives:
https://mthtrains.com/bc-0000217-0
It should work for my MTH Premier Santa Fe Hudson and Premier Santa Fe Texas, both made in the 2004-2005 time frame. Will that harness also work for my Premier GS4 Northern, which is a PS1 locomotive upgraded to PS2 3V? I'd like to buy a spare tether for each locomotive, just to have them on hand.