Third rail
The R156 is designed to navigate the clearances and tight turns of the NYC subway, with weight constrained to , so it can operate on elevated portions of the system. It joins a fleet of 62 diesel-electric work locomotives on the system. 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 John Deere Diesel drives a generator to provide auxiliary 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 the R160 subway cars. The traction motors are Siemens' SITRAC AC motors (the same ones on R160Bs 8843–9102). The R156 locomotives have provisions for the future installation of communications-based train control (CBTC) equipment, which were expected to be installed by June 2017.