The draw of the Clarksville Railroad bridge over the Cumberland River, mile 126.5, at Clarksville, shall open on signal when the vertical clearance under the navigational span is 47 feet or less. The draw shall open on signal if at least two hours notice is given when the vertical clearance is greater than 47 feet. The draw need not be opened for a vessel that arrives at the bridge more than 30 minutes after the time specified in the notice, unless a second two hours notice has been given.