With the rapid growth of fully automated urban rail transit lines, train autonomous operation systems based on train-to-train communication have emerged as the next generation of train control systems. These systems are characterized by safety, efficiency, flexibility, economy, and ease of deployment, featuring superior functionality, performance, and a streamlined architecture. However, they mainly lack extensive practical implementation experience. In particular, there is a lack of comprehensive comparative analysis regarding the impact of train-to-train communication systems versus traditional train-to-ground communication on the real-time performance of train spacing protection. Therefore, this paper conducts a comparative analysis of the two mechanisms in terms of train interval protection real-time performance. First, it introduces the architectures of the CBTC and TACS signal systems and establishes corresponding CPN models to explore the characteristics and applications of these two communication mechanisms in depth. Hence, through simulation analysis of metro lines, the paper compares the effects of both mechanisms on train interval protection real-time performance from the perspectives of turnaround intervals and movement authority update times. The research results indicate that train-to-train communication has significant advantages over train-to-ground communication in terms of interval protection real-time performance, providing important references for the design and optimization of future train control systems.