There actually is a size L power plug that it 5.0/2.5mm, so maybe the 5.5 would be just a bit too tight. Of course, the Type L plug seems to be a rare bird, probably why Lionel picked it!
FWIW, the power plugs on the command bases are the 5.5/2.5 size, that's what I needed to make my tethers for the TMCC buffer.
Right, there are plenty of 5.5 x 2.5 mm coaxial plugs in the marketplace. Apparently the difference of 5.0 to 5.5 mm (as the outside diameter is just enough to "not fit." I bought two Radio Shack coaxial plugs as "Size L" and they fit the jack on a Lionel FasTrack Terminal Track Section perfectly, but the problem is, its cover cap is too small to accommodate soldering the output wires from a Lionel 74 watt wall wort to the very small solder terminal under the cover cap. A case of "building a boat in the basement, then realizing that it's too wide to be moved outside and to the lake."
Regretfully, I cut the factory "size L" coaxial plug off the 74-watt wort so that I could attach the then-bare wires to a Lionel lock-on attached to tubular track. That worked fine for a while, but when I adopted FasTrack for dual circles at the base of the Christmas tree, I needed a "size L" coaxial plug. So I am my own worst enemy.
I sent a note to Lionel at their TALK TO US e-mail address and inquired about a "Size L" plug, but I haven't heard back from them yet. I wonder if "Size L" is proprietary to Lionel.
Mike Mottler LCCA 12394