EE AND CO-WORKER WERE ON THE TOP WALKWAY/PLATFORM LEVEL AT THE GRAVITY TAKE-UP (COUNTERWEIGHT) WHEN HE STEPPED OFF OF HANDRAIL DOWN ONTO MAIN WALKWAY PLATFORM, & STEPPED BACKWARDS, FALLING THR OUGH A 24"" X 30"" OPENING AND LANDED ON THE WALKWAY PLATFORM (APPROX) 14 1/2' BELOW. AMBULANCE TRANSPORTED HIM TO HOSPITAL WHERE HE WAS ADMITTED OVERNIGHT FOR OBSERVATION.