Wheel speed of heavy freight train locomotive under non-ideal traction conditions was measured with very high resolution and fine precision. Many authors published papers in this area over past century, but only recent advance in the information technology enabled measurement in such extent and resolution at the same time. Most modern locomotives have the speed sensor coupled with the traction motor. Unlike many other authors doing research in the area of wheel-slip, in the presented case the wheel speed was measured directly on the wheel, not indirectly on the traction motor. This approach made it possible to observe the wheel speed directly, without the requirement to implement motor — wheel model, which can introduce inaccuracies. Thanks to an independent method of train speed determination using a sensitive GPS receiver, wheel slip speeds were identified accurately regardless of synchronous slip phenomenon. Despite the adhesion theory, three types of operation mode were identified: the stable traction with negligible slip without oscillations, the semi-stable traction with evident slip and oscillations, and the prominent wheel slip. As the wheel speed was measured directly on the wheel, much larger oscillations than ever published were revealed. The source of the oscillations is discussed in the paper. A simple MatLab model of rotational masses was constructed and tuned to match the measurements. Eigenvalues and eigenvectors of the model are discussed, too. The model of rotational masses is completed with adhesion model to perform simulation. The simulation results are presented and compared with the results of measurement.