block system

plural block systems

1. rail transportOn railways, a system by which the track is divided into sections of three or four miles, and trains are run by the guidance of electric signals so that no train enters a section or block before the preceding train has left it.

   Signalling was by the standard 'block' system of the time, which prevents any two trains being on the same track section at once, but this was operated with boldness, with margins shaved. The Metropolitan would prove to have an excellent safety record nonetheless.

   2012, Andrew Martin, Underground Overground: A passenger's history of the Tube, Profile Books, →ISBN, page 41

2. mathematicsGiven a group G that acts on a set X, a partition of X that is G-invariant.