The ALCO Century 425 is a four axle diesel electric locomotive that has two thousand five hundred horsepower. The American Locomotive Company built only ninety one units of this model between 1964 and 1966. This locomotive was born from a special request by the Erie Lackawanna railroad which needed a strong machine equipped with a specific electric generator. The Century 425 is a direct upgrade of the older C 424 model and it gained one hundred extra horsepower. The Pennsylvania Railroad was the biggest buyer because they bought thirty one units for their fleet. Today it is a classic machine loved by train fans and a few units still run on tourist railroads or short line freight tracks.

The most famous slug operated next to the C-425 was the BU-41. The railroad built this unit inside its own shops by recycling the frame, wheel trucks, and traction motors of an old ALCO RS-3 locomotive (number 1554) that was retired.

They removed the main diesel engine, took off the engineer cab, and covered the top with a low metal hood filled with concrete and old rail pieces for extra weight.