  It is shown that co-linear injection of electrons or positrons into the
wakefield of the self-modulating particle beam is possible and ensures high
energy gain. The witness beam must co-propagate with the tail part of the
driver, since the plasma wave phase velocity there can exceed the light
velocity, which is necessary for efficient acceleration. If the witness beam is
many wakefield periods long, then the trapped charge is limited by beam loading
effects. The initial trapping is better for positrons, but at the acceleration
stage a considerable fraction of positrons is lost from the wave. For efficient
trapping of electrons, the plasma boundary must be sharp, with the density
transition region shorter than several centimeters. Positrons are not
susceptible to the initial plasma density gradient.
