Pollutant emission measurements under real operating conditions using Portable Emission Measurement Systems (PEMS) have been conducted for various groups of vehicles and machinery for many years. Since 2014, Real Driving Emissions (RDE) procedures were defined and implemented in homologation testing for Heavy Duty Vehicles (HDVs) and for Passenger Cars (PCs) and Light Duty Vehicles (LDVs) in 2017. In the case of Non-Road Mobile Machinery (NRMM), there is an ongoing effort to conduct real-world emission measurements; however, threshold values for harmful exhaust compounds have not yet been defined. This article presents an analysis of measurement results conducted for a rail bus equipped with two diesel engines. These six-cylinder engines have a displacement of 12.8 dm³ and an effective power output of 257 kW. Measurements were performed on a regular, non-electrified railway line between Krzyż Wielkopolski and Trzcianka, spanning a distance of 36 km. Additionally, the shunting operations of the rail bus at Krzyż Wielkopolski station were analyzed. During the study, emissions of gaseous compounds were measured using the Semtech DS analyzer by Sensors Inc., while particulate mass was measured with an AVL Micro Soot Sensor, and the particle number and size distribution were assessed with the Engine Exhaust Particulate Sizer by TSI. Due to the use of a rail vehicle, the testing procedures were designated as Real Train Emissions (RTE).