I decided the easiest way of finishing the underframe was to put a piece of plastic rod just behind where I wanted the channel to bend, soften said channel with liquid poly, and then bend into place.
I then attached another piece of I beam under the first by holding in a curved shape while letting the glue set.
Using the RT models height gauge which I use for all my stock, the central buffer/couplings were added (Narrow Planet).
I also added some pipe work to represent the braking.
With that done attention can now turn to the interior.