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Railroad bridges have been the topic of numerous threads, and the point is often made that they were mostly designed for the era steam locomotives.  And since steam locomotives have a higher loading than diesels or at least impose a higher dynamic load from the driver imbalance, most bridges remain overbuilt for today's diesel trains.It is true that bridge strength must be sufficient for the heaviest load, and the heaviest loads are locomotives.  But as I understand it, the fact that a bridge is designed to take a given load is not the end of the story.  The bridge also has a life.  The life is used up not only by deterioration from the elements, but also, by the number of times it is loaded, and the amount of each load.     The locomotive loading may have gone down, but the carload weight has gone up.  Freight cars have undergone a substantial increase in weight from an average of 80 tons gross in the steam era to 150 tons gross or higher today.  Moreover, the incidence of the car loadings outnumber that of the locomotive load by a large factor; perhaps an average of fifty-to-one.  I think what needs to be compared between the steam era and today, in terms of what is carried by bridges is not the weight of the locomotives, but rather, the total gross tonnage.  I am guessing that on average, that figure has increased many times since the steam era.
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