Compressible and Leaky Trains in Tunnels

It is hypothesized that phase differences commonly observed between computed and measured pressure histories in railway tunnels may be caused in part by reduced acoustic wave speeds in the annular regions of flow alongside trains. Such reductions in wave speed are shown to be a possible consequence of the compressibility of trains and/or the leakage of air into and out of trains. These effects are shown to be capable in principle of reducing the effective wave speed by a factor of (1-β)_1/2 where β denotes the train-tunnel area blockage ratio.

The potential influence of reduced wave speeds due to these causes is shown to be much greater in short tunnels than in long tunnels. An alternative explanation for the discrepancies observed in long tunnels has yet to be found.