Manual block systems work by dividing up a rail line into predefined "blocks", which are typically demarcated by fixed signs. Authority to occupy a block is granted by some sort of central controller, usually a dispatcher, who has the sole authority to grant such access. Before the widespread adoption of radio communications on trains, authority to occupy a block, along with other instructions, were communicated to trains much as timetable train orders were with station agents and trackside telephones. Some railroads, notably the Pennsylvania Railroad (PRR), had a system of manual block signals activated by wayside operators in stations or interlocking towers eliminating the need for some trains to stop. This manual block system is still on use on the Long Island Rail Road, which had been a subsidiary of the PRR.