Currently, there is no real “unused” redundancy available in the U.S. rail network. Any new redundancy that is built will be quickly utilized, otherwise railroads will continue to have to turn down new business in a growing economy.
What you want to avoid is putting all your eggs in one basket, wherein one major derailment results in systemwide catastrophy. Conversely, just because you have a section of trackage doesn’t mean you have to throw 25 mile long trains a day onto it just to make it pay for itself. This idea of capacity retrenchment during the 90’s was and is folly. The proof is in the freight that the railroads are willingly giving to other modes.