How to: Fix frayed wire on Lego 9v track connector
Heat shrink crimp connectors
Heat shrink tubing
Lighter or heat gun
Cut wire between track connector and ferrite core (black cylinder), cutting as close to the ferrite core as possible.
Cut wire at similar length on the controller end.
Strip back rubber insulation from controller end, track connector, and new wire.
Crimp wires together, sliding the heat shrink crimps as close to the connector as possible. DO NOT FORGET TO SLIDE HEAT SHRINK TUBING ON, IF YOU ARE USING IT. (I forgot for one half)
Use lighter or heat gun to melt shrink tubing, again sliding the heat shrink as close to the connector as possible.
Repeat for wire between track connectors if needed
*Disclaimer* There may be a better way to do this. I simply used what I had to fix the problem. No guarantees that this will work for you.
Thanks for reading and hope this helps someone,