My understanding: Many or most of the 737 fuselages going to Boeing can go through the [NP] steel through truss bridge in Renton, over the Cedar River, south of Boeing. Apparently the longest extended-body 737s cannot (or should not) go through that bridge (or some other limited clearance between Black River Jct. and Boeing); these need to come in from the north (I'd like to see a photo of a 737 on Wilburton trestle). In preparation for losing the line north of Renton, BNSF will replace the through truss bridge with a [through plate girder ?] bridge that will allow passage of all lengths of 737 fuselages.