This is absolutely true. It is standard operating procedure for the engineers to open the steamcocks upon departing Fantasyland Station. I hate to burst the bubble of those that think it's just for them, it's actually because the trains are climbing a grade from Fantasyland up to Main Street. When starting from a stop, especially on an uphill grade, locomotives are prone to wheelslip. Not only does wheel slip mean the train isn't actually moving efficiently (or at all), wheelslip creates excess wear on the driver wheels and rails. Most locomotives combat this with sand sprayed onto the track just before the wheels to increase traction. However, on the WDW locomotives, years ago the engineers were using too much sand, and again, causing excess wear. The sand tubes were cut and plugged at the footrail on each boiler. By opening the steamcocks the engineers can actually reduce the torque applied to the driving wheels via the steam cylinders, and have a larger margin for error to avoid wheelslip. So the steamcocks are opened for operational reasons unrelated to show. Again, apologies for bursting anyone's bubble.