49 CFR §237.5

For the purposes of this part— Bridge modification means a change to the configuration of a railroad bridge that affects the load capacity of the bridge. Bridge repair means remediation of damage or deterioration which has affected the structural integrity of a railroad bridge. Railroad bridge means any structure with a deck, regardless of length, which supports one or more railroad tracks, or any other undergrade structure with an individual span length of 10 feet or more located at such a depth that it is affected by live loads. Track owner means a person responsible for compliance in accordance with § 237.3.