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Nowadays telecommunication companies have shown a great interest in deploying broadband mobile wireless networks in high-speed trains (HSTs) with the aim of supporting both passenger services provisioning as well as automatic train control and signaling. The train carriage, as a confined steel structure, has specific propagation characteristics, which motivates the study of the indoor-to-indoor and outdoor-to-indoor radio propagation characteristics for broadband wireless communication systems in high-speed railways, constituting the main contribution of this work. This study has been performed by means of measurements considering an actual Long Term Evolution (LTE) network deployment, as well as a portable test transmitter and different configurations of antennas and receivers at 2.4, 2.6 and 5.7 GHz in a commercial high-speed rail line in Spain. The results show that radio waves incur obvious waveguiding effects inside the HST car. Moreover, for the propagation from the railway station to a mobile receiver inside the HST car, waves at higher frequencies experience less attenuation through the train carriage, by better propagating through windows. Although the railway station and train interior contain objects that induce a rich set of multipath components, the analysis of small-scale fading statistics shows that the channel still has a dominant path. Also, the LTE coverage tests for Base Transceiver Station (BTS)-Train and BTS-Mobile links were conducted and with internal and external antennas on board the train. We found that there was a strong signal penetration loss of approximately 26 dB caused by the train carriage structure. The final results constitute an initial model for the propagation incurred by a relay-based communications system for fourth generation (4G) network in railways. [ABSTRACT FROM AUTHOR]