When near-surface or near-bottom layers in the ocean are stratified, internal solitary waves (ISWs) of large amplitude can trap and transport fluid in their cores. The dynamics and energetics of a head-on collision of ISWs with trapped cores for a wide range of amplitudes and stratifications are studied numerically. The interacting stable waves of higher amplitude capture cores and carry trapped fluid in opposite directions. The interaction can trigger local wave instability of ISWs.