The EMD SD70ACU is a 6-axle 4,300 hp (3,200 kW) road switcher diesel-electric locomotive originally built by General Motors Electro-Motive Division as an SD90MAC, and rebuilt by the Norfolk Southern Railway and Progress Rail Services. They are operated by two railroads, Norfolk Southern and the Canadian Pacific Railway.

The SD70ACU began under Norfolk Southern, who rebuilt their SD90MACs to this configuration at its shops in Altoona, Pennsylvania. The first locomotive entered the rebuilding process in September 2015, which was number 7299. As of 2019, all 110 units have been released into active service.

All of the SD70ACUs will be rebuilt with the latest EMD cab which meets current FRA crashworthiness standards. The Siemens electronics will be replaced with Mitsubishi electronics. The rebuild also includes the installation of Ultra Cab II, locomotive speed limiter (LSL), and cab signals. The trucks will be rebuilt only as needed and they are not getting traction motors swapped out.

In 2018, Canadian Pacific started a similar rebuild program with their long dormant SD90MAC fleet. Unlike NS, the work on these rebuilds will not be done in-house, but by Progress Rail at their Mayfield, Kentucky and Muncie, Indiana facilities.

Norfolk Southern:

The units that were chosen for the Norfolk Southern rebuilding program are either ex-Union Pacific Railroad or from the CIT Group (CEFX) fleet of EMD SD90MAC locomotives. The SD90MAC was introduced in 1995 and was designed to use the new 16-cylinder, 6,000-horsepower (4,500 kW) H-Engine. However, technical problems with the 265 engine resulted in the locomotives being delivered with the 4,300 hp (3,210 kW) 16-cylinder 710G used in EMD's SD70MAC model. UP referred to these locomotives as SD9043MACs, and EMD planned to offer customers the option to upgrade them to the 6,000 hp (4,470 kW) 265 engine at a later date. EMD's upgrade program, however, was never carried out due to unresolved reliability issues with the 265. Over 400 SD90MAC locomotives fitted with the 4,300 hp (3,210 kW) 710 engine were built. Of the 309 SD90MACs Union Pacific had acquired, 100 units were sold to Norfolk Southern as part of the SD70ACU rebuild program. EMD was responsible for delivering the locomotives, and all units have been delivered as of November 2015. They are numbered 7229–7328. In July 2016, Norfolk Southern acquired another 10 SD90MAC locomotives from CIT Financial and the Indiana Rail Road for the continuation of the SD70ACU program. They will be numbered 7329–7338.

As of May 3, 2017, units that have received a full engine overhaul meet Tier 1+ emission standards, while units that have not yet received a full engine overhaul meet Tier 0+ emission standards. While all of the rebuilds are being upgraded to 4,500 traction horsepower, the Tier 0+ units generally only make 4,300-4,400 horsepower. Those units that have been overhauled and meet Tier 1+ emission standards are 7233, 7235, 7243, 7250, 7253, 7258, 7293, 7296, 7298, 7305, 7308, and 7319.

As originally built by EMD, the SD90MAC had a fuel tank capacity of 5,700 gallons. While owned by Union Pacific, the units started developing cracks in the frames around the fuel tank mounts. UP worked with EMD to solve the frame cracking problem by adding additional mounting brackets between the frame and fuel tank. In addition, the fuel filler pipes were modified to only allow a maximum of 4,700 gallons of fuel to be added.

As originally built by EMD, the SD90MAC had a maximum weight of 415,000 pounds. After being rebuilt in the SD70ACU program, the maximum weight initially increased to 420,000 pounds due to the additional weight of the new widenose and cab. Starting with the 35th SD70ACU rebuild, NS began adding additional weight to each end of the units to increase the maximum weight to 432,000 pounds. The first 34 units will also have the additional weight added as shop time permits. Those units that have received the additional weight are 7231, 7232, 7233, 7234, 7236, 7241, 7242, 7243, 7244, 7246, 7247, 7249, 7253, 7254, 7259, 7271, 7272, 7274, 7277, 7278, 7285, 7296, 7302, 7303, 7304, 7306, 7308, 7311, 7312, 7314, 7316, 7317, 7318, 7322, 7326, and 7328.

The original cab and electrical cabinet were replaced with new EMD SD70ACe style isolated cab AAR-style control stand, and electrical cabinet with Mitsubishi electronics. They retain their original TA22 main alternator and 1TB2830 traction motors. They are also equipped with a CCB II computer controlled braking system, the GE Ultra Cab II Cab Signaling, LSL (Locomotive Speed Limiter), as well as Automatic Engine Stop/Start. All units are equipped with NS LEADER/PTC (Positive train control) system and are equipped for use in distributed power unit (DPU) operation. A new electric parking brake has been added with brake wheel on the left side of the long hood. Ditch lights have also been added to the rear during rebuilding.