Later developments by national railway administrations included various levels of speed limitation and enforcement.Also, some systems were expanded to cater for speed limits for permanent or temporary speed restrictions.Railway signalling is the basic safety system controlling the movements of trains.It is the safety critical part of the train control function of the railway.
By designing the systems appropriately, it may safely be assumed that driver error and equipment failure will not occur simultaneously.
These have become known as Train Protection Systems.
Those systems that continuously monitor actual train speed and enforce adherence to a commanded speed pattern are referred to as Automatic Train Protection (ATP) systems.
Originally, the warning systems alerted the driver that he or she was approaching an adverse or restrictive line-side signal aspect and required him or her to acknowledge the warning.
Otherwise the systems would initiate a brake application after a short delay.