Zero-Emission Locomotives on U.S. Railways? Written by Alex Luvishis, Ph.D WaBTEC’S FLXdrive, designated as a BEL44C4D, is currently being tested as the middle unit in a three-unit consist, placed in between two Wabtec Tier 4 diesel-electrics. Based on emerging technologies in batteries, hydrogen fuel cells and renewable natural gas, zero-emission motive power is a possibility. In 2025, the U.S. EPA Tier 5 standard and the European Euro7 standard should come into effect. Both standards limit the emission of harmful substances by trains and locomotives to very low values. In particular, in the Tier 5 standard, in comparison with Tier 4, the value of NOx (oxides of nitrogen) decreases from 1.3 g/dHp-hr to 0.2 g/dHp-hr, and the value of PM (particulate matter) from 0.03 to 0.00. Thus, the standard requires zero PM emissions and almost zero NOx emissions.