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Abstract: Many railway turnouts are often installed near metro depots and stations, which lead to significant environmental vibrations reaching nearby infrastructure. Vibration in turnout zones can originate from various sources, such as rail joints, wheel-load transitions, uneven stiffnesses, rail corrugation, and small-radius curves. These factors contribute to turnout zones having considerably higher vibration levels than plain track sections. Additionally, in urban rapid transit systems, higher train speeds exacerbate wheel-rail impact excitation, further intensifying such vibrations. Despite turnout zones accounting for a large share of environmental vibrations, there have been few systematic studies on their specific sources and mechanisms in the context of rapid transit systems. This knowledge gap has hindered the development and optimization of vibration mitigation strategies for turnout structures. Therefore, in this study, we investigate five representative sets of turnouts from a rapid transit system in a Chinese city, with train speeds ranging from 80 to 150 km/h. Field tests were conducted on real operating trains, with vibration accelerations measured at turnout rails and tunnel walls. This study systematically examines the effects of turnout structure, train carriage position, speed, and vibration mitigation measures on the vibration source characteristics. Time-frequency methods were employed to analyze the test data. Our findings reveal that when train speeds exceed 100 km/h, leading and trailing carriages passing through turnouts induce low-frequency vibrations below 80 Hz, thus generating vibrations in the human-sensitive frequency range. Moreover, train-induced vibrations in turnout zones are primarily concentrated in three frequency bands: 0-20 Hz (associated with structural and stiffness irregularities in the turnouts), 50-80 Hz (P2 resonance of the wheel-rail system), and 150-200 Hz (natural frequencies of the rails).