This is an interactive single track swing bridge with an offset rotation point.
This actually existed on the Chicago, Minneapolis, St. Paul Railroad over the north branch of the Chicago River.
It was built on a sharp bend in the river and didn't need to open the normal 90 degrees. It was also quite long
for this design bridge which may explain the unusual method of support. Many thanks to Mike Kiehl (Etrains48)
for the pics and info.