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In the experiments and practical applications in a high-speed railway, it is observed that the carrier frequency of the sampled signal in a track circuit reader (TCR) is changed with train speed and goes beyond the upper permissive range prescribed for a jointless track circuit (JTC) in some cases. This can directly affect the availability of train target speed in train control systems and thus has an effect on the generation of the distance-to-go profile. It not only reduces the safety and efficiency of train traveling but also limits the improvement of train speed. To find the primary cause of the deviation in carrier frequency of the sampled signal in TCR (CFSST), this paper models the track-to-train continuous information transmission process using the transmission line theory based on the structures and principles of JTC and TCR. Then, the relation between the deviation in CFSST and the train speed is derived. Experimental results in high-speed railway have verified the correctness of the analysis, and the study can provides a strong theoretical basis for improving the safety level of railway traffic. Moreover, it can be a good reference for other countries where the similar track circuits are applied.