A puzzling film. The very sparse information that came with the film states that it concerns a third track being laid across the River Ouse, presumably the Skelton Viaduct (aka the Hook bridge or Goole railway swing bridge), in the 1930s. Yet it seems that this bridge has always had just the two lines going over it, so this may just be new girders arriving on the bridge to replace old ones.
This is one of a large collection of British Rail, and some pre- British Rail, films inherited by the track renewals company Fastline in 1996, and passed on to Fastline Photography when they folded in 2010. The Skelton swing bridge dates back to 1868, and is still in operation. It has a history of accidents involving boats bumping into it, and this might explain the arrival of these steel girders.