This letter studies the possible application of MIMO polarization diversity in road/railway tunnels. By means of an extensive measurement campaign extending from 2.8 to 5 GHz, different linear array architectures were studied, considering different polarizations within each array. Channel capacity was calculated assuming either a constant signal-to-noise ratio (SNR), whatever the array architecture, or a fixed transmitted power. Interpretation of the results is based on the spatial correlation between the incident electric fields on the array elements and also on the variation of the SNR in the case of a fixed transmitted power.