Right, I shall try and explain
First, here's a visual aid:
Each side is made up from 4 of these panels, a length of |_| shaped plastruct section and a cantrail bar.
No. 1 - Panels are marked out on 10 thou plasticard, as many as I needed plus some spares (used above). The windows are cut out whilst the sheet is still in one piece, then the individual panels are cut out giving you what you see as here. The pillars each side of and rail above the window should be 1.5mm (60 thou) wide, the one on the right needs some attention
No. 2 - On the front of each panel 40 x10 thou Evergreen strip is used to provide the outer framework, note how the window has an inner frame created at this stage 20 thou all the way around.
No. 3 - On the back 40 x 40 thou strip is used to create uprights each side of the panel, stopping short 1 mm (40 thou) from the top. The bottom panel is from 30 thou sheet (could easily be 40 if you wished), stopping short 0.5 mm (20 thou) of the bottom of the window. Note - for end of coach panels the 40 x 40 thou upright is left off as the coach end will take this function.
After enough panels are to hand, they are set out along the length of |_| section which forms the solebar, with a piece of 40 x 60 along the top inside the panels (40 thou edge to the panel). Both solebar and top rail stop short 1 mm from the end to allow the end to fit.
Over the doorways a length of 40 x 20 thou strip is then added to match the thickness of the panelling on the side and 10 thou side piece.
The windows have a 20 thou recess all the way around on the inside, allowing flush glazing once painted.
I hope this makes sense, I chose this method as it only involved cut-outs in 10 thou sheet, anything heavier and I struggle to get any sense of neatness. The 10 thou inner layer of the sandwich holds everything together nicely and the resulting coaches are actually fairly robust... though I wouldn'y advise standing on one...