I was rereading the article in the March 2008 "Railfan & Railroad" about MotivePower's MP36bbb (bbb = blahblahblah: there are numerous variants) commuter locomotives. The article describes them as having "semi-monocoque" carbodies: the side walls are doing stuctural work; they aren't just "cowl" units. (Good thing: the design is already excessively heavy (cf. the kerfuffle over the MBTA order), and roadswitcher designs need heavy frames, as witnessed by the extra framing Santa Fe had to put on its CF-7.) There are a couple of photos in the article of units under construction, and the framing looked as if it was very different in design from the sort of truss frame on an E or F unit. The classic carbody truss frame had a mixture of vertical and diagonal elements, just like the frame of a truss bridge. The MP36 seems to have mainly verticals: there is a diagonal in the "cell" right behind the cab and another at the extreme rear of the unit, but between these it looks as if the platform and the rail at the roofline are connected only by a "ladder" of verticals. I was taught that triangles were better for forming a rigid structure than rectangles (explaining the presence of diagonal elements in typical truss forms), so I was surprised by this. At a GUESS (note: guess-- this is speculation by a non-professional, on the basis of what was in the article) this means that the side walls of an MP36 are designed to carry a smaller propoertion of thetotal load than those of an E or F. Anybody here know more?
(Longer version posted to EMD forum.)
(Longer version posted to EMD forum.)
