The SC and the SW were built simultaneously at EMC, though EMC was pushing the SW which had a fabricated welded frame ("W" for welded and "C" for cast). The SC had a cast frame which EMC could not produce in its own shop, hence the attempt to sell the SW over the SC. Though there has been some confusion between the two models in the past, close-up photographs have revealed that the MoPac units were indeed SC's with cast frames (road numbers 9000-9003). When welded frames began in late 1936, many railroad people viewed them as a cheap imitation for a substantial cast frame. Consequently, engineers at La Grange decided to disguise their welded frames by applying a fake sheet metal section at each corner of the frame. These appeared on the SW1's, NW2's and TR2's until April 1949, when a cost cutting measure struck them from the package.By this time no one cared about whether frames were welded or cast. In 1938 MoPac subsidiary Union Terminal Railway (UT) got into the diesel act by purchasing an SW from EMC, which was outshopped as UT-5. In December the Saint Joseph Belt (SJB) picked up an SW (SJB-10).

In the roster section of the book, the following is shown for the SW's: UT-5 became SJB-5, then MP 6005, built 05/38 (c/n 718), retired 8/65, to PNC, to Pickens, renumbered PIRR-3. UT-10 is shown at becoming SJB-10, then MP 6006, built 12/38 (c/n 751) retired 9/63, scrapped. So by process of elimination, the locomotive shown MAY have been MP 6006, built as either SJB-10 or UT-10.

This was one of 60 (MP #'s 9000-9059) late production C36-7's ordered by the MP to UP specifications. The last ten of the order featured frames with anti-climbers originally destined for NdeM units but diverted because of that road's financial problems. These units also had a larger than usual (4500 gallon) fuel tank, forcing the air reservoir tanks into the radiator compartment. It rode on rebuilt truck bolsters from MP U30C trade-in's, and was rated at 3750 horsepower. Beginning in February 1988, all of MP's 9000 series C36-7's were upgraded at North Little Rock. The upgrade included rebuilt trucks, rebuilt engines, new radiators, and UP lettering.

One of the high-tech features of MP's C36-7 fleet is their use of GE's microprocessor-controlled Motor Thermal Protection Panel (MTP). This device analytically calculates the temperature of each traction motor's windings, using the ambient air temperature, amperage levels, and the motor's recent operational demands, and uses this data to establish the maximum current that can be fed to each motor should it enter its short-time rating. The MTP panel alleviates the need for the engineer to keep a close eye on the ammeter to avoid motor burnout.