by Homo sapiens
Along the track, overhead and undergrade bridges, as well as tunnels, have a reference number. This number usually indicates mileage to the 1/100th from wherever milepost 0.0 is (or was, e.g. Jersey City PRR to Philadelphia). But sometimes, the # is sequential (1,2,3,3A, etc.) What, if any, North American RRs still use sequential structure numbering, as is AFAIK still used today in the UK? I know NYC and DL&W did, and retained them through PC and EL until CR and the commuter RRs introduced mileage-based numbering. Farther back in history, other US RRs such as PRR were sequential; perhaps *all* were originally.