Following on from the previous post, here is a pic showing the elements that make up each side. On the left I have left the items spaced out, while on the right they are positioned very close to the final effect. You can sort of see what I meant about the possibility of getting it even slightly wrong so making the sides too short or too long, and also the possibilities of getting the side components slightly out of alignment or crooked.

The double-ended driving car had two more components per side (the cab side doors), while the single-ended driving car (motor or trailer, depending on period modelled) had one extra door per side - all that is rather academic if you build the resin kits from Radley Models as the body moulding comes with sides, ends, roof and internal partitions as one single casting.