Same basic bridge as the original but set up to be normally closed and open at the
apporoach of a boat that has been set up as a loco running on invisible track in
the water.
NOTE 1 - - - You must run invisible track across the span. If I included it as part
of the mesh a train crossing the span would also cause the bridge to open.
NOTE 2 - - - When the bridge opens it will not automatically stop an approaching
train. There are ways to do that involving markers, rules, etc.