In the pursuit of enhancing 5G signal coverage in high-speed train carriages, the IAB + 5G small-station approach has been proposed as a solution. However, the implementation of this approach may introduce interference to the existing railway networks, categorized as interference of the on-board 5G small station system on railway networks. This study primarily focuses on conducting an interference analysis of the on-board small station system on railway networks. Specifically, the theoretical values of spurious interference, blocking interference, and adjacent channel interference resulting from the on-board small station system on both the GSM-R and 5G-R systems are calculated. The analysis reveals that the spurious interference and blocking interference from the on-board small station system adhere to the standard requirements for both systems. Nonetheless, attention is required to address the adjacent channel interference to the GSM-R base station when the 5G gateway operates in the GSM900 frequency band. Future work should aim to resolve this interference issue, ensuring optimal performance and coexistence of the on-board small station system within the railway networks.