Something like this:
"Oncoming riders, welcome to Steel Venom. Please step into your car, pull down your harness and wait for an attendant. Leave any loose articles behind. Riders on the platform, take a step away from the gates in front of you. The gates are now closing. Keep your hands and arms in the car at all times and enjoy the ride."

-harness checks-

This next part started as:
"All right, we're ready to rock. Steel Venom's gonna strike in 3, 2, 1!"
And eventually morphed into:
"Launching in 3, 2, 1!"