108 pieces of chopped up styrene sheet later....
Next step is add long strips of 30 thou styrene along the outside edges of the two central longitudinal sections, effectively forming a "C" section, these will run across the tops of all the cross members, sort of replicating how the real vehicle was put together.
I think it will also be worth strengthening all the joints between the newly formed cross members and the edge rails by dropping epoxy resin into each of the internal corners. Then it's moving on to beefing up the framework outside the bolsters while also thinking about coupling arrangements, as the coupling hooks need to swivel to allow for end swing, I guess this will be a good idea - it's what the real ones did.
And I'm nowhere near the rivets yet!
Simon