UP enters partnership for new hybrid-electric locomotives

hybrid-electric locomotives Union Pacific Railroad (UP) and ZTR, a technology company and leader in locomotive control systems, signed an agreement to build hybrid-electric locomotives at UP’s North Little Rock, Arkansas facility. The first prototype is expected to be delivered in late 2023 and five additional vehicles are scheduled to be supplied in 2024.

The locomotives work much like today’s plug-in hybrid car, capable of operating in multiple modes with several ways to charge the locomotive batteries including wayside and onboard charging.

“Adding hybrid-electric locomotives to our fleet is another important step for Union Pacific. This pilot will help make our fleet more fuel efficient and further advance our commitment to reaching our sustainability goals, while testing the technology’s capability for expanded use across our locomotive fleet,” Union Pacific’s Senior Vice President of Engineering and Mechanical Shane Keller said.

Known as “mother-slug” sets, two locomotives are replaced with one diesel locomotive connected to an accessory or slug. In keeping with its sustainability commitments, Union Pacific converts older locomotives, typically with higher emissions. The slug unit will be converted and equipped with batteries, power electronics and controls supplied by ZTR. This helps achieve fuel and maintenance savings and offers environmental benefits allowing the hybrid-electric “mother-slug” to operate in single engine, battery charging or electric only modes.

“This investment aligns well with our ongoing goal to leverage ZTR’s technical expertise to support railways in their decarbonization and efficiency improvement efforts. This initial hybrid-electric mother-slug application will be the basis of our design for future hybrid-electric, long-haul and fully electric solutions,” ZTR Control Systems President Derek Shipley said.

As the new locomotives will be used chiefly for yard operations, the engineless slug increases traction motors available to the locomotive, boosting the pulling and braking power that is key for this kind of work.

Union Pacific began rolling out “mother-slug” sets with ZTR’s Nexsys control systems in 2017 and currently operates more than 65 in yards system-wide. Depending on the mode of operation, fuel savings and greenhouse gas emission reductions could total up to 80%.


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