Preventing train collisions is the first priority in train operation. For this reason, railway lines are divided into certain sections which only one train may occupy and in which no other train may be operated. This is the "block", and the section to be blocked is the "block section". The system which permits only one train to enter the block section is the "block system" and the devices used for the system are called "blocking devices". Fig. 4 describes the concept of the block section.
Fig. 4 Concept of block section
The types of block systems installed on JR lines are shown in Table 4.
The automatic block and cab signal block systems account for 73.3% of all lines, and these block systems are used on trunk and auxiliary trunk lines. Special automatic block systems are used on branch sections, and other block systems are converted to one of the above systems when installing modernizing facilities.