There will be no question of boats having precedence on the Stainforth & Keadby. A narrowboat or cruiser can easily stop within a few seconds. There will be no historic rights here - the S&K, like most canals, was eventually owned by a railway company which would have rescinded any historic rights if there were any (I don't know of any having been in place for railways that crossed canals, though rivers are a different kettle of fish). I have a vague recollection that the S&K actually converted itself into a railway company, seeing the coming thing, but I may be wrong.
We took our boat past this bridge a few years back. We just waited for the bridge-keeper to open up and signal us through. A couple of Pacers went through while we were waiting. Believe me, after the trip up the tidal Trent, any boater will be plenty happy to wait a few minutes here.
No, I believe that (many years ago) boaters were expected to wait for a gap between trains. Didn't Rolt go through that bridge in Narrow Boat?