Why are railroads so quick to abandon and remove track?

[LanceMiller77]

Why is it that railroads rip up track when they could simply leave it in case it needs to be used again in the future some time? I'm guessing that railroads (and industry) might be charged a tax on existing rail (is this standard across all of US and Canada?). Does the"salvage value"of the steel and ties make it worthwhile? Could a change in tax/heritage laws help curb this? ------