  We study the phenomenon of one-dimensional non-resonant tunnelling through
two successive potential barriers, separated by an intermediate free region R,
by analyzing the relevant solutions to the Schroedinger equation. We find that
the total traversal time is INDEPENDENT not only of the barrier widths (the
so-called "Hartman effect"), but also of the R-width: so that the effective
velocity in the region R, between the two barriers, can be regarded as
infinite. This agrees with the results known from the corresponding waveguide
experiments, which simulated the tunnelling experiment herein considered
because of the formal identity between the Schroedinger and the Helmholtz
equation [PACS numbers: 73.40.Gk; 03.65.-w; 03.30.+p; 41.20.Jb; 84.40.Az].
