A model of major ejection events in Galactic superluminal sources GRO J1655
-40 and GRS 1915-105 is proposed, in which episodes of pair creation give r
ise to the formation of optically thick super-Eddington outflow acceleratin
g protons to the limiting Lorentz factor of order 2-3. Probable source of p
airs is pion production in proton-proton and proton-neutron collisions whic
h may operate efficiently even for proton temperatures equal to several MeV
. Gamma-ray spectral signatures of this mechanism are pointed out. Subseque
nt deceleration of ejecta and formation of radio jets is analyzed in detail
within a shock deceleration model, The detailed data available for GRO J16
55-40 are shown to be in good agreement with proposed scenario.