are U41 twin block reinforced concrete, 2.4 m (7 ft 10 in.) wide, and weigh 245 kg (540 lb) each. They are equipped with hardware for Nabla RNTC spring fasteners, and a 9 mm (3/8 in.) rubber pad. (Rubber pads are always used under the rail on concrete sleepers, to avoid cracking). Next, a rail threader is used to lift the rails onto their final position on the sleepers. This machine rides on the rails just like the gantry cranes, but can also support itself directly on a sleeper. By doing this, it can lift the rails, and shift them inwards over the ends of the sleepers, to the proper gauge (standard gauge). It then lowers them onto the rubber sleeper cushions, and workers use a pneumatically operated machine to bolt down the Nabla clips with a predetermined torque. The rails are canted inward at a slope of 1 in 20. Joining track sections The sections of rail are welded together using thermite. Conventional welding (using some type of flame) does not work well on large metal pieces such as rails, since the heat is conducted away too quickly. Thermite is better suited to this job. It is a mix of aluminium powder and rust (iron oxide) powder, which reacts to produce iron, aluminum oxide, and a great deal of heat, making it ideal to weld rail. Before the rail is joined, its length must be adjusted very accurately. This ensures that the thermal stresses in the rail after it is joined into one continuous piece do not exceed certain limits, resulting in lateral kinks (in hot weather) or fractures (in cold weather). The joining operation is performed by thermite welding process which is equipped with a rail saw, a weld shear and a grinder. When the thermite welding process is complete, the weld is ground to the profile of the rail, resulting in a seamless join between rail sections. Stress in the rail due to temperature variations is absorbed without longitudinal strain, except near bridges where an expansion joint is sometimes used. Adding ballast The next step consists of stuffing a deep bed of ballast underneath the new track. The ballast arrives in a train of hopper cars pulled by diesel locomotives. Handling this train is challenging, since the ballast must be spread evenly. If the train stops, ballast can pile over the rails and derail it. A first