<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> This communication deals with the problem of multiple-input multiple-output (MIMO) wireless channels in rectangular-shaped tunnels. First, a reference scenario corresponding to the use of planar dense transmitting and receiving arrays is studied. The interest of such a canonical array configuration is that it is able to characterize intrinsic MIMO performances in tunnels, regardless of the number of array elements. Simple analytical formulas to calculate the channel capacity are proposed. However, in practical applications, the number of array elements is quite limited. Therefore, the capacity which can be practically achieved, either with optimized linear arrays or with uniform linear arrays, is discussed in the second part of this communication and compared to the results of the reference scenario. </para>