1. Introduction

Spain occupies the second place in the world, behind China, in kilometers of high-speed railway (HSR) built. The Spanish HSR that links the cities of Madrid and Barcelona was designed to travel at an average speed of 350 km/h in order to compete with the airplane. This implies an increase in average speed of 25% as compared to its predecessor, the railway that connects Madrid and Seville. Consequently, the dynamic forces exerted on the track and all its components have significantly increased.

One of the essential constituent elements that determine the quality of the track is the fastening system that connects the rails and the sleepers which, in turn, transmits both the static and dynamic forces exerted by the passing stocks to the railway infrastructure. Originally, the fastening elements were designed to prevent

the overturning and the transverse displacement of the rails. The advances in railway technology have compelled the fastening systems to fulfill new requirements such as the maintenance of the track gauge or the resistance to longitudinal loads due to the thermal expansion of the welded rails. In addition, the increased use of concrete sleepers has led to the need for the presence of an elastic element between the sleeper and the rail, the rail pads, to cushion the impacts and to reduce the high stiffness of the concrete sleepers.

Currently, the elastic fastening system between rails and sleepers is composed of metal spring clips, insulating plates of a polymeric nature and anchoring screws. These elements fix the rail to the sleeper minimizing the longitudinal and lateral displacements, as well as rotation, that are produced by the transversal, vertical, and longitudinal forces transmitted by the wheels of the passing vehicles. In addition, the fastening system provides the elastic response needed to counteract the vertical wave movements of the track that would give rise to high dynamic forces between wheels and rails avoiding vibrations and noise. Likewise, it maintains the gauge of the track and the inclination of the rails within the admissible tolerances avoiding the overturning of the rail.

The tightening of the fastener that holds the rail to the sleeper plays an essential role in guaranteeing the performance of the whole assembly. In this sense, the grip must provide a resistance to longitudinal displacements higher than the friction between the ballast and the sleeper; in addition, it has to reduce the movements of the head of the rail with a safety coefficient. At the same time, it must withstand the stresses to which it may be subjected to under in-service or accidental conditions. Finally, the electrical insulation between the two rails on electrified lines or equipped with signaling systems must be guaranteed.

Figure 1. Sketch of the VM fastening system [2].

## Optimization of Components of Superstructure of High-Speed Rail: The Spanish Experience DOI: http://dx.doi.org/10.5772/intechopen.80013

Among the existing types, the VM fastening system (see Figure 1) is the one installed in the Madrid-Barcelona HSR. Each of the components of this system (tension clips, guide plates, rail pad, plastic dowels, and steel screws) has been optimized regarding its mechanical behavior [1]. This chapter summarizes the studies carried out at LADICIM (Laboratory of Science of Materials of the University of Cantabria) over more than two decades, to improve the mechanical response of each of the constituents of the VM fastening system, in order to optimize the response of the assembly under in-service or accidental conditions. Likewise, the influence of parameters such as the tightening torque on the mechanical response of the system was studied. Finally, the deterioration undergone by the constitutive materials because of the action of external agents (humidity and temperature as a source for corrosion of metals or degradation of polymers) was analyzed in depth.
