Not only do the railroads have a different set of definitions for bridges and culverts than the highway guys do, those definitions now overlap with each other. The Federal Railroad Administration now defines a bridge as "any structure that spans an opening under a railroad track". That definition has an exception for culverts and drainpipes that are "located so far below the track that it only carries dead load from soil pressure and is not subjected to measurable bending, tension or compression stresses from passing trains". That keeps intact the original definition of a culvert:
Culvert: a tunnel or drainpipe structure that is entirely enclosed by soil or retained fill.
It looks to me like the current FHWA definitions of bridges and culverts are intended to separate those functions to simplify its National Bridge [Safety] Inspection Standards (NBIS, 23 CFR Part 650, Subpart C). When the FRA made its most recent changes (2010) to its Bridge Safety Standards (49 CFR Part 237), the agency expanded the definition of "bridge" to include any structure that would be affected by the dynamic loads imposed on the structures by a moving train. Therefore, that vague definition of a "bridge" now includes large culverts, unloading pits, railcar weight scale structures, as well as marine structures like wharves and piers. Furthermore, any culvert with a span over 10 feet is included in the bridge inspection requirements.
There are some huge culvert structures out there on the railroads. Many of them were originally arches constructed with loose brick or stones and include a bonafide "keystone" at the top. I've seen some masonry arch culverts that are say 9 feet wide, 15 feet tall and covered with more than 50 feet of unretained compressed fill. Those wouldn't qualify as a bridge in the current standard. Many of these culverts went over the original road and creek together. (In such cases, the roads needed to be raised up to meet the railroad and cross at grade when those roads were widened).
By the way, the original British definition of culvert was specifically for structures that carried wastewater (in many cases, extremely long structures down the middle of an alley or a street).
