The rapid growth of high-speed transit systems, such as High Speed Rail (HSR), is putting considerable pressure on TCP-based data transmission. It is well known that TCP is suffering from severe throughput degradation in high-speed mobility scenarios. The root cause at the transport layer however remains unclear and largely undetermined to date. In this paper, we aim to pinpoint the throughput bottlenecks and develop a throughput model to understand TCP in high-speed mobility environments. Based on the analysis of real-world HSR traces, we find that high-speed mobility will introduce significant challenges to the packet retransmission process after timeouts. And ACKs are more likely to trigger spurious retransmission timeouts in TCP flows in high-speed mobile environments. Such problems are not yet considered in the existing TCP models because classic timeouts can easily be recovered by retransmission in stationary scenarios. We therefore propose an enhanced TCP throughput model to integrate the above features. Our model analysis indicates that the optimization of TCP ACK latency is critical to obtain better throughput. Moreover, reliable retransmission mechanisms, e.g., multi-path TCP (MPTCP), can also bring notable benefits in high-speed mobility environments.