## 1 Introduction  
Railway communication systems are preparing for a significant paradigm shift with the standardisation of the Future Radio Mobile Communication System (FRMCS) [1] and the introduction of 5G to the railway sector. FRMCS opens the use of the mobile network to new applications such as signalling-critical data, Mission Critical Push-To-Talk (MCPTT) or Operational Cybersecurity. On its side, 5G allows the use of mobile technologies for additional services such as real-time CCTV, remote monitoring for maintenance works [2], among others. All of this is in a context where GSM-R is the technology used nowadays, which provides voice and data exclusively for signalling services [3] for train-to-ground communications.  
The proposed system includes new links to connect any railway element [4] while keeping the current train-to-ground communication. These links are extended to machine-type communications (MTC) [5] for track-side elements, train-to-train [6] and IoT for maintenance, as part of the new context of the Intelligent Transportation Systems (ITS) [7]. Nevertheless, this evolution implies a series of challenges to overcome regarding the implementation criteria, standardisation, spectrum regulation and legacy systems.  
In the current times, Mobile Network Operators (MNO) have begun to provide connectivity services to critical services in other industrial sectors [8]: emergency bodies, nuclear power plants, V2X or Industry 4.0 [9]. However, in the railway sector, the use of MNO is only restrained to transmitting non-critical data (CCTV, rolling stock logging collection or passenger information services).  
In this context, MNOs can play a key role in introducing MTC for critical services such as signalling systems, audio recording data collection, or operational telephony [10]. This alternative solution is especially interesting for low railway traffic lines, goods and the mining industry, given the high costs that private fibre optic deployment requires nowadays [11]. In addition, the proposed solution allows the modernisation of rural railway lines, allowing the installation of modern signalling systems and avoiding the human factor in telephonic block tracks.  
This paper assesses the feasibility of implementing 4G/5G public networks to transmit critical railway signalling services. The methodology is based on measurements mainly to establish a realistic characterisation of the communication parameters for railway signalling systems when introducing MNO networks. Particularly, the approach includes three scenarios: first, an interlock (DXL) and an object controller (ObjC) are interconnected via a developed prototype for this purpose; secondly, the resulting signalling traffic is simulated in a telecommunications laboratory; finally, field measurements are performed to assess the behaviour in a real railway environment. The objective is to obtain sufficient data to define the radio and network Key Performance Indicators (KPIs) to provide adequate service to the railway signalling systems. In order to achieve this goal, the prototype has been developed to collect information from the protocol stack perspective, being connected to a signalling equipment mock-up.  
This document is organised as follows: first, Section 2 describes the methodology followed in this study; secondly, in Section 3, the measurement set-up explains the measuring method carried out together with the prototype constructed for this purpose; then, Section 4 describes the measurements gathered in the three scenarios; finally, Section 5 presents the conclusion obtained from all this work.