Photo by James Lalande, ONR North Bay station, April 2000.
Here's an SD50F. These locomotives might seem quite odd to anyone who's not used to them, but this is, in fact, nothing but an SD50 in a different carbody.
The concept of enclosing the walkways in a so-called "widebody" was once very appealing to Canadian railroads for added protection of the locomotive's machinery and the crew from the elements. It certainly must have seemed nice for the crew not to have to walk through a blizzard to check on something. However, such a design eliminates rear visibility altogether.
When CN bought its first widebody locomotives (with the Bombardier HR616 in 1982), William L. Draper, CN's then-chief of motive power, requested an important modification. The units were to have an angled cutaway behind the cab for rear visibility around curves. This cutaway was thus named the "Draper Taper". Such locomotives have rear windows on their cabs, and of course the Draper Taper, visible in this photo on the second unit just behind the flanger sign. The taper starts just below the horn on the first unit.
Like CN's other GM units, these have snowshields for the locomotive air intakes, 1st generation widecabs with 4 windows, ditchlights, nose-mounted headlights, big honkin' snowplows and anticlimbers and a bell between the number boards near the cab roof. The bell is mounted this high (rather than the American location just ahead of the fuel tank, down at truck level) because it was thought that icing problems in the winter would be avoided. However, CN's new C44-9W's, SD70I's and SD75I's have the bell mounted in the low location and the crews assure us they do not freeze up in the winter.
Note that, due to the unusual carbody, the grillwork is different. The air intake for the dynamic brake is at the same level as the engine air intake, unlike conventional SD50's and SD60's. The horn is also in quite a unique location, placed to avoid blasting the eardrums out of the crew. Also, since there are no external walkways the cab requires a side door, steps and handrails like an F-unit. F-units have the nickname "covered wagon" and so do these widebody units. However, unlike the F-unit, the body does not form part of the structure of this locomotive - it is merely a "cowl" or hood over a regular locomotive. This makes maintenance much simpler. If any large parts need to be taken out, the cowl can be removed.
CN has 60 SD50F's and 63 (originally 64) externally-identical SD60F's. The SD50F's have a 645 engine with a horsepower rating of 3600, making them "SD50 +"'s according to the locomotive enthusiast magazine Extra 2200 South, so they should really be "SD50 + F"'s. The first 3 SD60F's read "SD50AF" on the builder's plate, but the 3800 horsepower 710 engine really makes them "SD60F"'s.
Other Draper Taper widebody units:
the Bombardier HR616, built for CN in 1982
the GM SD40-2F, built for CP in 1988
the GE Dash 8-40CM, built from 1990 to 1993 for CN, BC Rail and Quebec, North Shore & Labrador
The widebody design has since fallen out of favour. It limits rear visibilty, even with the Draper Taper, increases noise (imagine being in a metal can with a howling locomotive prime mover) and increases cost due to engineering changes made by the locomotive manufacturer to standard production units.
There's a closeup of the trucks here.