>>1434694Largely a function of history. The very earliest road diesels were boxcabs, which gave way cab units. Full width bodies were important for streamlined passenger trains, and there wasn't really a reason to change the design for freight uses, since any switching would be handled by smaller diesel switchers or old steam engines. But after WWII, as dieselization took hold, there was a trend toward standardized, multipurpose locomotives instead of ones for every task. This was especially important for branch lines and small railroads, as they needed to be able to both do switching work and haul trains of considerate size and speed. This was impetus for the invention of the road switcher: larger than a yard switcher, but retaining the hoods instead of full body, primarily for visibility, but also ease of maintenance (cab units tended to have the body as an integral part of the frame, unlike hood units). ALCO was the first to build these, and after EMD's unsuccessful attempt to built a cab unit with improved visibility (the BL-2), joined in with the GP and SD series. GE would join the market, and ALCO would exit, but little would change until the late 1980s or so. By then, there were more than enough old Geeps running around doing road switcher duties, and six-axle units were increasingly used almost exclusively as road units, doing almost no switching. Crew comfort and safety became more important, with expanded/wide cabs being mandated in Canada, before becoming standard issue in the US. The push for greater fuel efficiency meant bigger radiators for dynamic braking, and expanded electronics cabinets behind the cab. Visibility is no long an issue, since they don't switch or run long hood forward, so it is only ease of maintenance that keeps the hoods narrow otherwise. They don't run passenger service, so an interior connection is unnecessary. Modern passenger locomotives, however, are usually full-width body for this reason (but there are exceptions).
