EMPLOYEE WAS RIDING IN A PORTAL BUS TOWARD HIS SECTION. THE PORTAL BUS OPERATOR WAS STOPPING FOR A LOCOMOTIVE AHEAD OF HIM BUT HE DID NOT SEE THE LOWBOY COUPLED TO THE LOCOMOTIVE. THE PORTAL B US RAN INTO THE LOWBOY AND THE EMPLOYEE RIDING IN THE LOWBOY STRAINED HIS LOWER BACK AND NECK WHEN IT SUDDENLY STOPPED.