The MP54AC (also known as MP40PHTC-T4) is the latest locomotive in the MPXpress family and the only model available for sale in the US. It is designed to both meet the EPA's stringent Tier 4 emissions standard and offer higher performance than the MP40PH-3C. The MP54AC uses a pair of Cummins 16-cylinder QSK60 engines rated at 2,700 hp each (5,400 hp total), which qualifies it as the most powerful diesel-electric passenger locomotive in North America, both currently and historically (The 8600 hp Siemens ACS-64, an electric locomotive, is the most powerful passenger locomotive overall). During periods of low power demand, the locomotive can operate on just one engine to reduce noise pollution and boost fuel efficiency. Currently, the MP54AC can be built as new or by having existing MPXpress locomotives rebuilt to the standard. The locomotive competes with the EMD F125 and Siemens Charger.
GO Transit was the launch customer for the MP54AC. The first prototype unit was built by converting a MP40PH-3C owned by GO Transit (unit 647). MotivePower removed the EMD prime mover and HEP motor and replaced them with the twin Cummins engines. Heavy modifications were made to the body to accommodate extra air intake and exhaust stacks. Unit 647 was delivered to GO Transit late 2015 and was seen under testing on December 12, 2015
Initially GO Transit had planned to convert a total of ten MP40PH-3Cs to the new MP54AC standard; it later ordered 16 additional newly built MP54AC locomotives and eventually converted the order for rebuilt units to new builds for a total of 26. The MP54ACs are also planned to replace the remaining F59PH locomotives left on the roster.