The present study focuses in the propagation and deformation of the first compression wave generated when a high speed train enters a tunnel. This first wave is a determining factor in shock wave formation inside the tunnel, and therefore in sonic boom phenomena. This is modelled with the one-dimensional equations for the flow in a tube, and an analogy that relates a piston inside a tube with the entering train, both opening the door to an algebraic formulation that delimits the problem of shock formation. The model is represented in a characteristic form to find the relation between the distance where the pressure wave becomes a shock wave (named the regression distance) and the parameters that define the initial wave profile, namely the maximum pressure increment and the maximum pressure gradient downstream the entry portal. Later, steady and unsteady wall friction and heat transfer effects are analysed. Differences between the regression distance for the cases with friction and heat transfer and without them, seems to be delimited, that makes the algebraic formulation suited for fast decisions at the time of conceptual design of high-speed lines. Model validation with more complex models and experimental data is provided.