It's probably trucks that is largely responsible for the condition of highway bridges, and thus rail is a solution - and a cheaper one than spending on highways.  It is also that many highway bridges were cheaply constructed in the first place.
 
There has been a lot of mention about how rail bridges have stood stout for a hundred years and highway bridges are falling down, but in fact early rail bridges were cheaply built as well, usually tall spindly wood affairs and there were many bridge collapses in the 1800's.  It was the rebuilding of the network from the 1890's-1920's that was done to very high standards for which we can be thankful.