Bringing cellular capacity into modern trains is challenging because they act as Faraday cages. Building a radio frequency (RF) corridor along the railway tracks ensures a high signal-to-noise ratio and limits handovers. However, building such RF corridors is difficult because of the administrative burden of excessive formalities to obtain construction permissions and costly because of the sheer number of base stations. Our contribution in this paper is an unconventional solution of mmWave fronthauled low-power out-of-band repeater nodes deployed in short intervals on existing masts between high-power macro cell sites. The paper demonstrates the feasibility of the concept with an extensive measurement campaign on a commercial railway line. The benefit of using many low-power nodes with low-gain antennas compared to a baseline with only high-gain macro antennas is discussed, and the coverage improvement is evaluated. Based on the measurement results, a simple path loss model is calibrated. This model allows evaluation of the potential of the mmWave repeater architecture to increase the macro cell inter-site distance and reduce deployment costs.