One day there was a large swivel bridge, which crosses a wide river. On almost everyday, this bridge body parallel with the river so that ships from both sides of the bridge can across the river to sail freely. But at the certain times, the trains always come through this place and the bridge was installed across the river so that trains can across the river. The bridge guard was in a small post on one side of the river, where he can operate the control of the bridge so that the bridge could be rotated and mounted in place when the train comes.
