The EMD SD45 was the responce to GE's U36C and achieved 3,600 hp from a massive 20-645E3 prime mover. This locomotives' prime mover proved troublesome. The 20-645 could break its own crankshaft. This caused some railroads to still order SD40's through the SD45's production, as they did not find the added 600 hp worth the added expense. However the locomotive did not go un purchased. Southern Pacific, Santa Fe, Great Northern Railway, Union Pacific and the Northern Pacific Railway all bought SD45's to varing degrees. The SD45-2 was designed to remedy some of these problems. 3,600 hp SD45's are rare in 2013, Montana Rail Link still rosters a few, and uses them daily. Other regional and shortline operators continue to use the units. The first production SD45, GN #400, is named Hustle Muscle. The units is currently owned by the Great Northern Railway Historical Society, the unit is operable.
The SD45-2 was an initial success as well; consisting of being a complete improvement or upgrade as part of EMD's "Dash 2" line, it had little to almost absolutely no flaws during the early stages of production and during the model's official introduction. Though it mainly proved to be popular with the Santa Fe (ATSF), Southern Pacific (SP), and the predecessors of CSX (Seaboard System and Chessie System). A cabless variant or version ("booster" unit) was built as well, though it was primarily ordered and built upon request from the ATSF (as a result of the railroad's successful demonstrations and trials with their own rebuilt units). Though popular and compatible with the original SD45 as well, it's large 20-cylinder engine unfortunately didn't have long-lasting warranties like the 16-cylinder SD40 (and subsequent SD40-2); meaning that the 20-cylinder variant of the 645E3 engine wasn't guaranteed to last nearly as its 16-cylinder counter-part.
Though such railroads which owned fleets of their "45" series diesels built from EMD (such as the F45) were favored over their "40" series diesels, the "40" series proved (in later years) to have completely outlasted EMD's "45" series as a result of requiring less maintenance, consuming less fuel, and having better warranties with their engines. But by the time of the introduction of the SD50 (and GE's "Dash 8" line), owners began to retire and rebuild their fleets of SD45 and SD45-2 units into more compatible 16-cylinder types to be compatible with their newly-produced fleets of units (such as CSX; though, they eventually traded-in their remaining SD45 and SD45-2 units for SD50's). Though, the ATSF simply downgraded and de-rated their fleets of "45" series units, and weren't even retired until long after the BNSF merger in 1995-1996. Thus, making the ATSF the last owner and having the largest fleet of original "45" series, 20-cylinder types or models of EMD diesel locomotives on any North American or US Class 1 railroad.
SD45-2 units lasted somewhat longer than most SD45 units on various owners rosters, yet they were all eventually replaced with succeeding types of six-axle EMD (and GE; though not until around GE's track to dominance in 1985) models beginning with the SD50. Thus, having SD45-2 and subsequent models (including the original SD45) withdrawn from revenue service by 2005 from every other North American Class 1 railroad.
What was unique about the SD45's design, was its flared radiators (to whom the SD45-2 doesn't have); which was the first (and one of the only) types of diesel locomotives built by EMD to have such a unique design (excluding the GP40P-2 and other specialty units), and the original SD45 is the only official diesel locomotive built by EMD to have flared radiators. The others being rebuilds from other companies (aside from the later model designated SD70I/SD70AC units having flared radiators).
There are also several different versions of the SD45. Such as:
SD45X (Experimental, 4,200hp)
SD45T-2 (Part of "Tunnel Motor" series)
SDP45 (Modified passenger version; E-L units classed as SD45M's)
SD45-2B (Cabless booster units rebuild by ATSF)
SD45-2u (Sub-Versions by ATSF)
SD50M-3 (Rebuilt cabless version of the original SD45-2B)
DDM45 (Narrow-gauge eight-axle Brazilian version)
SD44R (SP experimental version-only SP unit rebuilt with 16V-645 prime mover)