page_title: UK railway signalling
section_title: Permissible speed indicators
hierarchical_section_title: UK railway signalling / Speed indicators / Permissible speed indicators
context_page_description: The railway signalling system used across the majority of the United Kingdom rail network uses lineside signals to control the movement and speed of trains.
  The modern-day system mostly uses two, three, and four aspect colour-light signals using track circuit block signalling. It is a development of the original absolute block signalling that is still being used on many secondary lines. The use of lineside signals in Britain is restricted to railways with a maximum speed limit of up to 125 miles per hour. This is the maximum speed at which the train can travel safely using line-side signalling; if the train runs any faster, it will not be possible for the train driver to safely read colour-light signalling. Trains operating at speeds faster than 125 mph use an in-cab signalling system that automatically determines and calculates speed restrictions.
context_section_description: In the UK, every section of railway line has a maximum speed, known as the Permissible speed.
  Table A of the Network Rail Sectional Appendix provides a list of these. Where there is a change in permissible speed on a line, a permissible speed indicator sign will show the new speed. Train crew are expected to know every permissible speed for all the routes they work over as part of their Route knowledge. When the speed limit increases, the rear end of the train passing must completely clear the speed limit sign before the speed limit can take effect. This minimises the risk of derailment.
  Permissible speed indicator
  Permissible speed indicator. This example is displaying a maximum permissible speed of 125 mph (201 km/h).
  Differential permissible speed indicator
  The number below the line always shows a higher speed and applies to passenger trains, parcel and postal trains and light locomotives, while the top number applies to all other trains. This example therefore is displaying a maximum permissible speed of 70 mph (110 km/h) for passenger trains, parcel and postal trains and light locomotives, in addition to a maximum permissible speed of 40 mph (64 km/h) for all other trains.
  Diverging permissible speed indicator
  This example is displaying a maximum permissible speed of 40 mph (64 km/h) only for the diverging route to the left. Unless otherwise indicated with another sign, this does not apply to the current route.
  Warning indicator
  Warning indicators are provided whenever there is a reduction in permissible speed of a third or more, and are placed at the service braking distance from the start of the lower speed.  They are colloquially known as 'Morpeth boards', after the 1969 derailment at Morpeth, Northumberland that led to their introduction.  Often, but not always, there is an AWS magnet placed before the indicator to give an audible warning to the train driver. 
  Diverging warning indicator
  These indicators warn the driver of a lower speed limit on a route diverging ahead. This example is displaying a maximum permissible speed of 75 mph (121 km/h) on the left diverging route.  These warnings are provided whenever there is a reduction of permissible speed of a third or more, and, again, are placed at the service braking distance from the start of the lower speed.
caption: Permissible Speed Indicator