Justin,
Your set is a "K-Line by Lionel", which is why it has a tether. In fact there are two of them, as you're probably aware, one between the Powered 'A" (lead) and the 'B' (middle), and the other between the 'B' and the non-Powered 'A' (trailing).
Because there are no easy-to-find wiring diagrams for "K-Line by Lionel" we have two options:
1.) Replace the broken tether with a new one.
This is part no. 620-1521-325, as seen below, and the Lionel Parts website indicates that it is identical in both the 'B' unit and the Non-Powered 'A'. It would be easy to open up the whichever unit has the bad tether and replace it:
Unfortunately Lionel Parts online lists this part presently as "unavailable".
2.) Use your remaining good tether to determine where the two broken wires in the bad tether should end up in the connector they pulled out of.
Because they're identical you can fix the bad one to replicate the exact connections you see on the good one.
Good luck. I hope this helps.
Mike
