Several companies are now supplying precast subway units to speed Great Britain's modernization program. Two main types are in production: precast box sections, and precast lidded-U's. The precast boxes are 5 feet long, 8 feet square and weigh 9 tons. Top internal corners are frequently beveled to accommodate lighting conduits or other services. Precast lidded-U's are mainly used for larger subways. The subway units are installed on a binding layer of concrete, usually of 4 inch minimum thickness. Accurate, tight jointing or units is essential. Joints are either of the lap or tongue and groove type. In place connection between units can be by successive bolting or by welding together protruding reinforcement. Joints are then grouted solid, and vertical reinforcement is added as necessary.