What he said. Solenoid spring-return mechanisms "fail-open."
To get the slow "Tortoise-like" gate motion I'd think you'd need a geared motor which means you need power to slowly/prototypically return the arm(s) to the open position. I can't imagine any crossing gates with "battery-backup" or the like to provide several seconds of power to slowly raise the gates so that it returns to the open position if power lost!