In this context, "abandonment" simply means that the railroad has stopped running trains over the line. It is not legally the same as "property abandonment"... it's an abandonment of service. The railroad may or may not rip up the tracks -- there are a bunch of abandoned lines in New England that haven't seen a train in 20 or more years but still have rails in place under the weeds, like the B&M Central Mass branch. But whether they rip up the rails or not, they still own the real estate and have the same legal rights over its use as any other property owner, until they choose to give up those rights through sale or lease.
"So are you saying that the owner of the tracks owns it until they lease it or sell it?"
Yes.
"Does that mean that if a track is abandoned for 100 years but the railroad that owned it still exists never leased or sold it still has the right to sue you if you drive a locomotive on it? "
Well, the hundred year example gets into other legal areas like "adverse possession"... if, for example, you build a driveway that crosses a corner of your neighbor's land and he doesn't complain about it for a period of time set by law (I think it's 20 years in Massachusetts, but don't trust my memory on that), then by default you acquire rights to continue to use the land. Also, land titles sometimes get confused over time due to sloppy surveying, unpaid liens, and other circumstances. Finally, there are cases where the railroad never owned the land to begin with but simply had an easement, meaning a right to use it for a specific purpose (like running trains), and rights to the land would have reverted to the original owner (the local farmer or whoever, or his heirs) when the trains stopped. Consult an intro legal reference on property law if you're interested in details on that sort of stuff... the same rules apply to a railroad right-of-way as to a house lot.
But basically, if the Fitchburg RR owned a piece of land in the 19th century, then the B&M got title when it took over the Fitchburg, and they still own it unless there's been some disposition.