Radio Electronic Token Block is a system of railway signalling used in the United Kingdom The Signaller issues an electronic "token" by radio which is received and displayed by special equipment in the train's cab. This token is the driver's authority to occupy the single line, and he is then given permission to pass a "Stop Board" and enter that section. The Stop Board is used instead of signals and therefore needs no electrical supply. The whole line can be operated by one Signaller and needs very little infrastructure othern than the track itself, making it a cost-effective method.
More information about RETB can be found in this article http://www.signalbox.org/branches/aj/index.htm about the resignalling of the Cambian Line in Wales