The EMD DDA40X is a 6,600 hp (4,922 kW) D-D locomotive, built by EMD from 1969 to 1971, exclusively for the Union Pacific Railroad. It is the most powerful diesel–electric locomotive model ever built on a single frame, having two 16-645E3A diesel prime movers. Union Pacific has marked DD40X on the cab exteriors, while EMD literature inconsistently refers to this model as either DD-40X or DDA40X.