Preliminary experimental results have shown that MIMO techniques can also be applied in tunnels environment, despite the small angular spread of the direction of arrival/departure of the rays. However the gain in capacity compared to a SISO transmission is strongly dependent on the tunnel size and on the communication range. The interpretation of this result, based on a modal approach of the propagation in a rectangular tunnel, is given. Considerations on wave polarization are also introduced, polarization diversity being a solution for reducing the size of the mobile array.