Movable bridges enable the bridge to be moved to allow other traffic to pass.  This is accomplished by vertically lifting the deck (a vertical lift bridge), by rotating the deck (a swing bridge), or by raising the deck at an angle through the use of weights (a bascule bridge).  The original movable bridges were the drawbridges of medieval times, raised by ropes and pulleys.  As early as the sixteenth century Leonardo da Vinci designed a movable bridge for military purposes.  Among the more familiar bascule bridges is the drawbridge, in which one leaf or two opposing leafs are raised to allow marine traffic to pass underneath.  The modern era of the bascule bridge reportedly began in 1893 with the Van Buren Street Bridge in Chicago, a rolling lift bridge built on a design patented by William Scherzer (1858-1893).  Some are being replaced today with fixed bridges at higher clearance, to eliminate the delays caused in accommodating marine and vehicular traffic.
 
     The Cape Cod Railroad Bridge (a lift bridge shown here) was built between 1933 and 1935 over the Cape Cod Canal in Massachusetts.  The center span is 544 feet, with a 136 foot clearance over the water.  The bottom right image here shows a train on the bridge.  The images show the bridge in various stages of lift; it is maintained in the raised position to facilitate marine navigation until it must be lowered for an approaching train.  Prior to this bridge, there was a double track Strauss trunnion bascule bridge, built in 1910 over a narrower canal. 












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