BR&P wrote:If there are structural issues with the bridge, they need to be dealt with. The bridge does not know how many trains are going through Rochester, or whose train is crossing it.
Physics does come into play here. More trains equates to more tonnage, which in theory creates repeated stress on critical joints. Nothing lasts forever, and the repeated weight of a passing train should create wear and tear.
Indeed, this is part of the reason why NS is replacing the Letchworth viaduct. In addition to operating heavier cars and increasing the speed limit on their new bridge, it will allow NS to run more trains across it. (Whether more trains will actually happen down there is best discussed elsewhere.)
BR&P wrote:And if somehow my hypothetical scenario were to be put in place, I have no doubt repair or replacement of the structure would be accomplished BEFORE the other route was cut.
With that being said, repair could potentially take the bridge OOS, which will create problems for the LA&L if it takes more than a few days. As stated previously, building a replacement bridge next to the existing span has the challenge of obtaining land from the Genesee Valley Greenway and we all know how cooperative the trails lobby is with the RRing industry.