An analytical model is proposed for calculating the influence of separation behind the nose of a high-speed train on the compression wave produced when the train enters a tunnel. The viscous drag associated with the back flow over the train and over the adjacent tunnel wall produces an aeroacoustic dipole whose strength increases linearly with the distance of the train nose into the tunnel and accounts for the linearly growing tail of the compression wave observed in model scale and field measurements. A detailed comparison with experiment suggests also that the contribution to the compression wave pressure rise from the large vortex ejected from the tunnel portal when the nose passes into the tunnel is in practice likely to be small, contrary to previous predictions that have modelled the exit flow as a uniform parallel jet exhausting from the tunnel.
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