Since the opening of the Japanese Tokaido line, interest in high speed rail travel has greatly increased. In some countries, the Japanese example of building an entirely new track has been followed, for example the 250 km/hr Rome-Florence line in Italy (5). In others, notably the U.K. with its 200 km/hr High Speed Train and later its 250 km/hr Advanced Passenger Train, emphasis has been placed on the development of vehicles capable of achieving the high speeds on existing track, subject to only minor upgrading. As a result, it is necessary to consider the modification of existing tunnels as well as the design of new tunnels when dealing with the increased travel speeds. The emphasis of the present paper is placed on such modifications, but the discussion is also directly applicable to the economic design of new tunnels.

The difficulties involved in providing adequate ventilation in long tunnels are considerable. On the other hand, the effects of pressure wave action are relatively slight. In contrast, when a tunnel is short, ventilation is a far less demanding problem, but pressure wave action can be of great importance. Pressure waves are typically generated when the front and rear of a train enter or leave a tunnel. Each wave passes along the tunnel at approximately the speed of sound, reflects at the opposite portal, and travels back along the tunnel, only to be again reflected. Trains therefore travel through an environment in which sudden and large pressure changes occur.