  Usually tunneling is established after imposing some matching conditions on
the (time-independent) wave function and its first derivative at the boundaries
of a barrier. Here an alternative scheme is proposed to determine tunneling and
estimate transmission probabilities in time-dependent problems, which takes
advantage of the trajectory picture provided by Bohmian mechanics. From this
theory a general functional expression for the transmission probability in
terms of the system initial state can be reached. This expression is used here
to analyze tunneling properties and estimate transmissions in the case of
initial Gaussian wave packets colliding with ramp-like barriers.
