Deploying mobile relays in public transportation is a simple yet effective way to avoid the electromagnetic insulation within vehicles and to increase the Quality of Service (QoS) perceived by passengers, which is sometimes low. Mobile relaying can be done by using an LTE/EPC 100%-compatible architecture. However, this solution induces extra-overhead. In this paper we evaluate the QoS of an LTE mobile relay architecture for public railway transport systems for two representative services: client-server requests and voice communications. We compare the performance of a direct transmission against a mobile relay architecture for different types of requests and different load conditions. This work, therefore, evaluates the mobile relay performance in terms of load time, throughput, packet loss ratio and end-to-end latency. Our findings show that a mobile relay architecture highly improves the QoS performance for train passengers. Furthermore, the gain is greater as the load increases.