Alternate double single track proposals for low demand high-speed railway lines

Multiple track lines are the only satisfactory solution for high speed railway lines when demand is high. However, in several countries double track high speed lines are being built for low demand lines, which are very expensive and inadequate solutions. This paper presents alternate double-single-track (ADST) lines as an alternative to doubletrack lines. The idea consists of using single-track where the infrastructure is very expensive (tunnels and viaducts) and double-track where it is cheaper (smooth orography) combined with small changes in departure times so that trains may cross in the double-track segments with no reduction in travel times. The solution is shown to be very efficient for intermediate traffic demands and the construction and maintenance costs are substantially reduced (close to 40%). A linear programming program is given that (1) decides the optimal sequence of single and double-tracks to be constructed and (2) optimizes the timetables for a given demand. The method can also be applied to combine high speed tracks with conventional existing tracks, where the conventional ones are used mainly for train crossing and circulation of slow and freight trains. These solutions produce very important savings (close to 90%) with very reasonable travel times. The lines Palencia-Santander in Spain, the Santiago-Valparaíso-Viña del Mar in Chile and the Dublin-Belfast in Ireland are used to illustrate both proposals and to demonstrate that they are very interesting alternatives in some practical cases.