An analytical and experimental investigation is made of the compression wave generated when a train enters a tunnel fitted with a long, uniform hood with a rectangular window. The window is situated at the junction of the hood and tunnel, which are taken to have the same uniform cross-sectional area. An understanding of the mechanics of this canonical configuration is important for the design of tunnel entrance hoods for new high-speed trains, with speeds in excess of 300 km h$^{-1}$. The compression wave is formed in two stages: as the train nose enters the hood and as it passes the window. The elevated pressure within the hood produces a flow of air from the window in the form of a high-speed jet, whose inertia generates an additional rise in pressure that propagates into the tunnel as a localized pulse. Multiple reflections from the window and the hood portal cause the temporary trapping of wave energy within the hood (prior to its radiation into the tunnel). All of these aspects of the flow are modelled analytically and the results are found to be in good accord with new model-scale measurements and flow visualization studies reported in this paper.