It's not uncommon for wires in the tether to break right at the connector in the tether to break, I keep a bunch of those in stock for that very reason. FWIW, I don't recall ever having the break any other place in the tether. Typically, I can diagnose these on the rollers by wiggling the wire back and forth at the connector.
The locomotive connector will occasionally have a pin break free from the solder as well, normally those can just be resoldered.