the one with the large "rear porch" is the SD40-2, which is not exactly the same as the SD40..
actually, there are four variants in the locomotives you are talking about, you probably worked on all four:
- built 1966 to 1972, most retired by the 1990's, very few still operating today, probably none on Class-1 railroads.
- built 1972 to 1989 - one of the most successful diesel models of all time, many still operating today.
- built 1965 to 1971 - most out of service by the 1990's, a few still solder on today, although many have been rebuilt
and are no longer technically SD45's, but many still retain their unique SD45 carbody..
- built 1972 to 1974 - the least common of the four, very few still in service today, and probably none in original form,
as most that are still running have been rebuilt.
The SD40-2 is, by far, the most common, and that's the one with the large rear porch.
The SD45 is easily identified by having the flared radiators. (which the SD45-2 did not have)
Today, only the SD40-2 remains still in service in large numbers, on the Class-1 railroads..
SD40 and SD40-2 have 16 cylinder engines, SD45 and SD45-2 have 20 cylinder engines..
there are other differences..both Conrail and CSX rostered all four models:
http://paintshop.railfan.net/images/eby ... h2-cr.html
Note the longer front and rear "porches" on the dash-2.
the SD40 and SD40-2 are quite visually different, the dash-2 being noticeably longer, and with the
pronounced "porches" that the SD40 lacks.
SD45, the "flared radiators" at the rear are an easy SD45 spotting feature:
http://www.railpictures.net/viewphoto.p ... 816&nseq=6
SD45-2, no flared radiators:
http://www.railpictures.net/viewphoto.p ... 96&nseq=10