Compo C/BB/K brake block is a rigid moulded friction material, grey-black in colour, having a random fibre asbestos base and containing metallic particles. It is manufactured only in the form of brake blocks, integrally moulded to a back plate. This material was originally developed to have the ability to withstand the heavy braking demands of Electric Multiple Unit (EMU) stock, operating suburban passenger services. Its use has since been extended to a range of duties from low speed shunting locomotives to high speed passenger trains.