Automatic Train Protection (ATP)
The subsystem within the overall train control system which automatically ensures compliance with or observation of some or all speed restrictions or movement authorities’. Normally, the term ATP refers to the provision of failsafe protection against collisions, excessive speed, and other hazardous conditions which may arise during train movements by preventing trains from ignoring control commands. This definition covers what could be described as ‘Comprehensive ATP’ (see below). Less effective systems (such as TPWS, AWS and Trainstops) are sometimes also classified as ATP. As a result, the following hierarchy of functionality is proposed, with ATP as the ‘global’ term: Warning Systems ‘systems assisting observation of movement authorities, based upon manual activation’, e.g., the Driver Reminder Appliance (DRA); Automatic Warning Systems ‘systems automatically assisting observation of movement authorities’, e.g. the standard British AWS system; Automatic Train Stop ‘a system automatically enforcing compliance with the limits of movement authorities’; Partial ATP ‘a system automatically enforcing compliance with speed restrictions and movement authorities at some locations or for some vehicles’; Comprehensive ATP ‘a system automatically enforcing compliance with all speed restrictions and movement authorities (for all vehicles) within a given area’. This type of system is often divided into two subcategories, Intermittent ATP and Continuous ATP. There are many different types of implementation but all require the transmission of control data using track circuits, inductive loops, balises or radio signals. Radio signals can be diffused by broadcast or leaky cable feeders.