…why limit yourself?
I'll bet your very first reaction to T-Trak, the first time you saw it, was this: those sure are small modules. Well, yes they are. That's part of the beauty of T-Trak, but it's also a limitation. Not much room for an industrial spur, let alone a passing siding, in 12-1/8" of length. So you think about building a double. That's a little better. No, wait, maybe that's a little too long…it's gotta fit on that shelf in the garage between shows (or whatever).
Remember, T-Trak is a modular standard based upon another modular standard: Kato Unitrack itself. Straight sections of Unitrack are manufactured in multiples of 62mm. Take the longest available straight, 248mm, connect it to a short one, 62mm, subtract about 1/32" from each end for clearance, and there you have it: the "standard" T-Trak module length. Why not 248mm + 124mm instead, if that suits your purposes better? It's just as easy to build a module to any multiple of 62mm, and there's no law that says you can't.
Worried about mixing such oddball modules into a show layout filled with standard lengths? You have three ways to handle the situation:
1) Build a pair of modules to the same odd length. For show layouts, they go back-to-back on opposite sides of the loop.
2) Plan your oddballs so that they may be combined to a multiple of the standard module length. (This is the approach I took with my Feasible Flats W, Feasible Flats E, and Little Chenango River Bridge modules.)
3) If the layout design has a leg with a single, linear run of modules capped by a reverse loop, you're all set. (Our club does such layouts, such as the photo at right.) Just make sure everything fits on the table.