[1] The U25B (nicknamed U-Boat) is the first commercially successful domestic diesel electric road locomotive designed, built, and sold by General Electric after its split with the American Locomotive Company (Alco), a company dating back to the steam era.

The U-Boat put GE on the road to becoming the top locomotive producer in the U.S., much to the chagrin of EMD.

It introduced many innovations to the U.S. diesel locomotive market, including a pressurized car body and a centralized air processing system that provided filtered air to the engine and electrical cabinet, thus reducing maintenance.

Southern Pacific 3100 is now on permanent exhibit at the Orange Empire Railway Museum in Perris, California.

Later numbered SP 6800, it became a goodwill ambassador for the railroad in 1976 when it was painted in an elaborate red, white, and blue color scheme in celebration of the nation's bicentennial; it was later renumbered and repainted in standard SP livery and was donated to the museum.

The 3100 is fully certified to run on any of the nation's railroads and is frequently used as motive power for offsite work trains.Several more examples survive, all in varying states of preservation: