## 1 Introduction
The current WSN technology can be used as a permanent monitoring system and considerably reduce the cost of installation and maintenance since no wiring is required. However, a permanent sensor platform is expected to have a long life span, especially if economic viability is taken into account. In this sense, it is very important to analyze the characteristics of the components of the WSN because its behavior will determine the response of the whole system. This paper presents our experiences in the process to select a sensor platform which meets the system requirements. Efficient energy consumption sensors, efficient communication and energy harvesting systems have been studied to guarantee the requirements in both the slab installation phase and the maintenance monitoring stage. In addition, we also believe our approach can cope with the network coverage problem and tackle the transfer of large quantities of data reliably. This second advantage with respect to the aforementioned wired systems, has already been highlighted in one of our previous papers [7]. That work centered on describing how sensor nodes are deployed along the infrastructure, forming clusters, taking periodical readings about the structural health and sending the information to trains passing through. The application scenario was simulated using Cooja [8]. Now, in the present paper we focus on the selection and implementation of the real sensor platform, describing, in more depth, the different sensors and energy devices used, showing several results obtained from the multiple field evaluations we have carried out. The sensor platform presented in this paper can be applied to any kind of railway infrastructure, but the prototype we have developed is especially tailored to be used in a slab track system. Slab tracks are currently being studied as substitutes for traditional ballasted tracks for railways due to their advantages such a high stability, no maintenance and long life cycles, but at the expense of a higher price. Our sensor platform is designed to be part of the slab tack, being inserted inside it during its construction, so that it can be used in both installation and maintenance phases. The slab track becomes an active element capable of monitoring and reporting information about the environment such as vibration performance, distance and inclination and also assisting operators in the installation phase. The experiences described in this paper are part of the project entitled Fastrack [9] funded by the Spanish Government's FEDER program. The main objective is the design of a new slab track system for high speed trains (faster than 250 km/h) that is environmentally and economically sustainable. In this context, a real time monitoring of the system is important to meet the project requirements. The project is currently testing the system in laboratories of the Spanish government institution for the study and experimentation of public works, called CEDEX. Other technical objectives are:  
* Address an affordable and environmentally sustainable manufacturing. * Have elements that reduce the production of noise and vibrations from railway traffic. * Low maintenance, increasing the hours of availability of the infrastructure operation. * Quick and easy repair if necessary, avoiding long route cuts  
The rest of the paper is organized as follows: in Section 2 related work is presented. Section 3 presents a description of the system requirements. The platform, its implementation and evaluation are described in Section 4. In Section 5 the evaluation of different energy harvesting mechanisms is presented. Section 6 shows the cost analysis of the Sensor4 PRI platform. Finally, Section 7 concludes the paper.