I dun'no what you were worried about. Now I see what you were talking about the logo panel. Being a sheetmetalworker, it took particular notice in that prototype picture you had of the fact that that long strip was actually made up of several shorter lengths and it also appeared to be fixed to the internal framing at quite close intervals. Even when sheetmetal is laying flat, it is not flat in any sense. Being roll formed, there are intrinsic tensions within the sheet. Hollows and humps are the norm. Folding the edges, such as that logo's panel would have, helps to draw some of that out. Our models of locos, coaches and cars are flat for one reason only, the molds they are formed in have had their shapes formed by milling machine, which can guarantee flatness that the eye cannot fault.