EMPLOYEE WAS A PASSENGER IN #23 MAN BUS THAT HAD STOPPED AT THE HAND SLAP FOR #3 DERAIL WHEN IT WAS STRUCK IN THE REAR BY #27 MAN BUS. ON IMPACT THE EMPLOYEE STATED THAT HE HIT HIS HEAD ON THE BULK HEAD OF THE PASSENGERS COMPARTMENT AND THEN WAS THROWN FORWARD INTO THE DECK OF THE COMPARTMENT.