Why are rails left even when they are no longer used?

This question came to me as I walked around the Santa Fe Rail yard and along some of the walk ways the rails are still in the ground with the just wides filled in with gravel or asphalt. Is it because its not worth the effort to pull them up? Another interesteing odidity that comes to mind is in Albuquerque there is a major 4 lane road, Paseo Del Norte, that the tracks cross almost perpendicular, there is no cross buck but it looks like if they moved the jersey barriers a train could still use the tracks as they have the typical at grade crosssing for cars and trians. Whats going on there? I dont see any practical way they could use those tracks without causing a traffic nightamare. Any insight from someone in the rail industry would be appreciated thanks!