What I use is MTH tethers, you can get them in straight line or 90 degree models. They're 10 wires, so plenty to do the upgrade.
I will warn you about the Lionel Reading T1, the place you'd like to put your tether for a straight shot out of the tender is almost an inch of solid diecast material! I worked for the better part of an hour with a Dremel and the carbide cutting bit to hog out for the locomotive connector. I'm not sure I'd do it the same way if I were to do it again!