Prior to track laying it is important to install drainage that will prevent the railway from becoming waterlogged. This takes the form of drainage pipes and carrier drains, as well as in some areas, attenuation ponds (a pond designed to slow the passage of water from surface run-off to the ground / drainage system).
Preparation of formation
The ground on which the track is to be laid is first prepared and compacted to the required stiffness to ensure there are no soft spots and, where appropriate, treated to prevent the upward movement of clay-like material into the ballast (as this would eventually cause a deterioration in the ballast with a consequent effect on track quality).
Once the formation is prepared, a layer of ballast is spread to the required depth and is then levelled to a high degree of accuracy.
Concrete sleepers are used to hold the rails in place. These are spaced out very accurately using a machine designed specifically for the purpose, ensuring that they are correctly aligned prior to the offloading of rails. Approximately 95,000 sleepers will be used on the Borders Railway.
Offloading of Rail
The rail will be transported by train from the manufacturer on specially designed wagons. Each train will carry twenty-four 108m long rail sections. These will then be offloaded from the end of the train in pairs, and placed directly in the sleeper housings. As each pair of rails is offloaded, the rails will be secured to the sleepers to allow the train to proceed a further rail length to repeat the process. At a later stage, the rail is welded up to give “continuous welded rail” and is stressed to prevent the rail from buckling in high temperatures.
The Borders Railway will have 7 new sets of points, at the ends of the passing loops, and at Tweedbank station. Most of these will be very large, to allow a fairly high speed for train running. The points will be transported from the manufacturer in sections, and reassembled on site prior to the long rails being installed.
Once the rail is in place a train conveying more ballast, using wagons known as Autoballasters, each carrying 64 tonnes of ballast, will travel along the line. A train of 30 Autoballaster wagons, weighing about 2,900 tonnes, will discharge the ballast through hoppers along the track in about two hours. The ballast discharge is controlled remotely while the train is driven at slow speed, to ensure even distribution of the ballast. The top ballast holds the sleepers firmly in place, and a ballast “shoulder” on the outside of the sleepers, maintains the track geometry.
Once the top ballast is in place, the track is compacted using a high capacity tamping machine. This works along the track sleeper by sleeper, lifting the rail and sleeper and packing the ballast underneath using “forks”, to bring the rail to its design line and level, to ensure smooth running of the trains. This process is repeated two or three times.
Finally, the ballast on the sleepers will be tidied up using Automated Finishing Machines (AFMs).