# Realisation of a Novel Functionally Redundant Actuation System for a Railway Track-Switch
## 1 Introduction  
### Literature Review  
The railway track switch system, which allows trains to change between routes, is a safety-critical part of the rail network. Track switches are required to be highly reliable as a single fault will block the switch and the routes of which it is a part. Current switches are based on designs, which are unable to tolerate a fault in the actuator system. Existing or traditional track switch systems, work by sliding the switch rails laterally over slide chairs using electro-mechanical, hydraulic and/or pneumatic actuators. The two switch rails are connected by a stretcher bar and driven from a single actuation force while switching [1], and are secured in place (once switched) with a mechanically operated lock. These systems are prone to failure and any failure of the switch system causes delay and occasionally catastrophic derailment as happened recently in the US where two people were killed and injured ninety others [2]. In a recent study, COMSA Rail Transport company showed that the number of failures in the Switches and Crossings (S&C) varied between 1.4 and 2.5 per year per S&C in Madrid-Sevilla line in Spain for the years 2010-2016 [3]. [PERSON] et al. [1] analysed the failure data in the rail network in the UK and showed how track switches limit the performance of the rail network because of the design limitations and lack of advanced controller development in the switch system. Several pieces of research have been carried out to improve the reliability of the track switch system by using condition monitoring approaches [4; 5; 6] and introducing fault tolerance concepts [1]. Recently, several new track switch projects such as REPOINT [7], S-code [8; 9; 10], In2 Rail [11], and Winterproof Turnout [12] are emerging to develop new concept switches for improving rail network performance. Of all of these, the REPOINT approach to redundant actuation is the only one to have been tested in a demonstrator at full-scale, (as part of a project REPOINT-Light, [13; 14; 15]), which will be discussed herein. [PERSON] et al. [16; 17] showed that including redundancy into the switches system can improve the capacity and the reliability of the network. As a result, one important approach is to redesign the switch system using a new actuation mechanism. Unlike conventional one-dimensional (1D) lateral track switching, the new mechanism, works in 2D, lifting the switch rails from their locked position and moving in a semi-circular path into the other locked position [18]. The full switch concept proposed the use of this 2D actuation mechanism, multiple actuators for redundancy and a stub switch layout (see Figure 1a). The authors claim [17] that the stub switch has benefits over a conventional switch including that the rails are full section throughout, without the tapered points. However, in discussion with stakeholders and funders, it was decided that developing a switch with a novel mechanism, multiple actuation, and an unconventional track system might be achieved in a series of steps. Hence, the work we will describe here uses only the novel actuation and multiple actuator bearers and applies them to a conventional switch panel to prove the functionality of the new actuation in a railway environment. Figure 1 shows the difference between the layouts of \"REPOINT\" and \"REPOINT-Light\" systems. One of the design key points in the fault-tolerant switch is that the locking system is no longer separate; it is passive and part of the actuation subsystems. In conventional switch systems, two different systems for locking and detection are used. The detection mechanism (displacement sensors in most of the switches) detects the position of the switch and if the switch rails are in the desired position, the locking system (mechanical or hydraulic) locks the switch rails and restricts any movement of the switch rails. In the  
Figure 1: Different layout for full REPOINT and REPOINT-Light systems (**a,b**). Numbered elements: 1. Stock rails, 2. Movable switch rails, 3. Common crossing, 4.