The article provides an analysis of the procedure for conducting and results of research tests of a traction geared motor box (hereinafter GMB). The traction GMB is provided for installation on a motor trolley as part of an asynchronous electric drive of a suburban electric train. Railway transmission systems operate in an environment with various sources of mechanical and electromagnetic excitations, where the possible unstable operation of the traction motor is one of the most critical indicators. This paper describes the structure and functionality of the physical test equipment and a model of a coupled system with a complete electrical circuit, including a rectifier, a DC link, a traction inverter, a motor-gear unit, and a system of mechanical connections, including flexible damper couplings and shock absorbers. This approach makes it possible to determine the intrinsic resonant frequencies of the GMB design, as well as the level of vibration activity of the GMB under dynamically changing loads. The obtained cascade characteristics of vibration activity correspond to both a long-term steady-state mode and transient mechanical disturbances during start-up, a change in rotation speed and the transition of the GMB to the run-out mode.