Railroads don't want to get rid of disused or underused lines. They want to get rid of uneconomic lines that don't have a reasonable likelihood of ever getting better. As with BNSF holding onto the Stampede line then later putting it back in service, UP undoubtedly has some idea of a case when Tennessee Pass might be needed again - and you don't throw away a perfectly good main line over a Rocky Mountain pass. NWP, OTOH, would have been long gone absent SMART and NCRA (alias Your Tax Dollars) because there's no reasonable prospect for serious freight traffic in that area; though it's not impossible for a small-scale short-line freight operation to at least break even with local traffic because their cost structure is much lower than UP's.