The MPI MP8AC-3 is a  work train locomotive built for the MTA. It dubbed the R156. The MTA ordered 28 units in 2006. After some difficulty with the  first prototype, the first unit, numbered OL912, was delivered in May 2012. Deliveries continued through 2013. Almost all have been delivered.

The R156 is designed to manage the nothing clearances and tight turns of the NYC subway, with weight constrained to 112,000 lbs so it can operate on elevated portions of the system. It joins a fleet of 62 Diesel-electric work locomotives on the system. Mainly GE 70-tonners. Third rail power is not used because the third rail is normally turned off at work sites for safety reasons. Instead, the R156 is powered by a 6-cylinder, 800 horsepower Cummins QSK 23 Diesel engine. A separate, smaller Caterpillar Inc. Diesel drives a generator for head end power, and to heat the main engine and keep batteries charged when the main engine is not running. Trucks are supplied by Kawasaki, which provides the same wheel and traction sets for new R160 subway cars. The traction motors are Siemens' SITRAC AC motors. These units meet EPA Tier 3 standards.