train 10 includes two diesel locomotives 18 20 pulling a large number of cars 22 along the track as the train approaches an entrance end 24 of the tunnel a cover installation 26 is operated to close the exit end 28 of the tunnel specifically as the train approaches the entrance end of the tunnel it trips a track circuit or switch 30 which is located far up track from the exit end and also up track from the entrance end of the tunnel and which is coupled by a cable 32 to a control 34 of the cover installation the control 34 can cause a cover 36 to be moved from an open position to a closed position over the exit end of the tunnel so as to prevent the free flow of air out of the exit end of the tunnel fig2 illustrates the installation of fig1 with the cover 36 in a closed condition over the exit end 28 of the tunnel and with the train 10 positioned with its front end within the tunnel as the train moves progressively deeper into the tunnel it tends to compress the air in front of it the pressure buildup is appreciable because the locomotive 18 fills a considerable portion of the corss sectional area of the tunnel such as on the order of 50 if the exit end 28 of the tunnel were open to the free outflow of air therefrom there would be a small buildup.