U1D100

U1D100 refers to a specific model of the **U1** series of diesel-electric locomotives produced by the **Union Pacific Railroad** (UP). This locomotive is part of the **U1D** subseries, which was introduced in the late 1960s as part of Union Pacific’s modernization efforts. The "U1D100" designation indicates it is the 100th unit produced in the U1D series, though it is also sometimes used colloquially to represent the entire U1D model.

The U1D locomotives were built by **General Motors Electro-Motive Division (EMD)** and were among the first of Union Pacific’s fleet to feature the **Dash 2** design, which included improved aerodynamics, a more streamlined cab, and enhanced reliability. These units were powered by a **16-cylinder 645E3** diesel engine, producing approximately **3,000 horsepower**, and were equipped with **D12A** traction motors. They were primarily used for heavy freight hauling across Union Pacific’s extensive network, particularly in the western United States.

The U1D series was notable for its durability and efficiency, serving Union Pacific for decades. Many of these locomotives were later modified or repowered as part of Union Pacific’s ongoing fleet upgrades. Some U1D units were also transferred to other railroads, including **BNSF Railway**, following mergers and acquisitions. While not as numerous as later models like the **SD70ACe**, the U1D100 and its siblings played a key role in the railroad’s operations during the mid-to-late 20th century.

Today, a few U1D locomotives remain in service, though most have been retired or converted into static displays or museum pieces. Their legacy endures as part of Union Pacific’s historical fleet, representing an important era in railroading.