I was asked a question that I wasn't sure of the answer to.

Why are abandoned tracks sometimes pulled up, but other times left in place?

For instance in New Jersey, the local railroad line was abandoned east of Mount Holly in the late 70s by Conrail, and in 1984 the tracks were all ripped up between there and points east to Toms River (the old PRR "Back Road"). Almost all of the branches associated with this line are also totally removed. Only a few bridges and abutments give any indication that a railroad was ever there.

Abandoned by Conrail around the same time was much of the original Camden & Amboy alignment, the tracks of which are very visible running along Route 130 through much of central New Jersey. They only become completely removed through Hightstown. Similarly, the tracks that were once part of the CNJ's mainline to Atlantic City through the pine barrens is still there, rails and all.

Does the railroad have to remove the tracks if completely abandons the line (sells the RoW property)?
Does the railroad leave the track in place just in case of future traffic? To prevent people from building on the RoW?
Does the Railroad have to removed abandoned track for environmental reasons?
Does abandon track get re-used on other lines? Is it worth its scrap value?
What obligations do the railroad's have to their abandoned infrastructure?

I'll be interested in the answer. The last trains ran on the Royal Gorge Route in the late 60's, but the track is still there, all the way from Canon City clear up to Leadville by way of Buena Vista. For a long time, UP stored equally abandoned coal cars on the tracks until people complained that they were an eyesore and dripping rust into the river.

Now the cars are all gone but the tracks are still there, rusting away a little more each year.

First thing is to be sure what you think is abandoned really is. Just because it is out of service does not mean abandoned. It can not be abandoned legally without the approval of the Surface Transportation board. The rail is worth taking up for scrap or relay depending on the condition. Another thing not readily visible may be buried underground cables or pipelines. The railroad gets paid yearly fees for this and if abandoned that stops. It might also have to do with revisionary clauses. Many lines were built on rights of ways with a clause that the land would revert to the adjacent land owners if it was removed. Some lines went back and purchased the right of way outright. (These deeds are very hard to run down). The Rails to Trails laws were to ensure the right of way would always be there if needed for rail service but this has become a real nightmare in the few cases where the railroads needed the right of way back, resulting in the trail folks filing suits to prevent the railroads from relaying the rails, and costing the railroads millions to defend their actions because the courts won't throw the suits out outright. Then eminent domain must be used to purchase the right of way.
Charlie

Another reason the line may be mothballed subject to revival even as a long term storage track for obsolete freight cars pending sale to a scrap company.