A signalling system where trains are kept separated by the signalling of each block.

In urban areas, blocks are short to reduce headway and thus increase the number of possible train movements for peak operation. Urban blocks typically rely on starter signals to leave a station, automatic signals between adjacent blocks, and home signals to enter a station. 

In rural areas, blocks are long (sometimes just one single block between stations or passing loops) as the traffic there is less dense. These blocks rely on a starter signal to enter the block and a home signal to enter the station (or passing loop).

Starter and home signals are absolute signals and directly controlled by the signaller. Automatic signals are permissive signals and typically operated via track circuits (track circuit signalling).

Block separation is much safer than time separation and much more costly to implement.

See also time separation.