The Tomytec approach is really pretty good. Plaza Japan has a lot of their mechanisms already, and they have many different frame lengths and at least three different truck wheelbases available, and sideframes are separate. The only 'problem' is that the frames are designed for horizontal motor mounts. If you're looking for a typical roadswitcher hood, you need to mount the motor on edge. So I copied their either frame approach for the truck holes and pickup strips, with a cradle to relocate the motor. It's not particularly complicated, and if you were doing a one-off, you can do the entire deal yourself in thick styrene. I made mine into masters and cast them into resin, but that's not necessarily part of the solution unless you're making a lot of them.
I've had excellent results with these trucks, and I've evolved into gearhead motors on the one-truck drives. If the flanges are too high for you, the are split-axle design and its easy to pull them apart, chuck them in a Dremel, and turn the flanges. No big deal.
I think a Shapeways-style printed metal frame is possible, just never seen it. I wonder about the tolerances in areas like the bearing blocks and also fitting an insulated motor cradle - the pin dimensions on those are rather precise.
