What do railroads do with their old telegraph lines, since they don't use telegrams anymore. They're still in fair condition on the MBTA's Fitchburg Line, which tells me that they're still being used for something.
By the time the telephone came around, most telegraph lines were re-purposed into telephone lines. Today in some places they'll be used for signals, switches, and other railroad electronics, and in other places they stand derelict, even though they still hold tension in the wires so they may still look perfectly functional. On the Fitchburg line like you mentioned, I don't think they're used, although in many areas (Ayer for example) they have other wires lower on the pole that are in use. Lines abandoned 30 years ago can still look perfectly fine if the wires were tensioned correctly and the poles last long enough.
On the Stony Brook branch in Westford, there's some places where the wires look perfectly intact, nice, tight, looking like they were just installed yesterday, while in other places the poles are falling over and the wires are all over the ground or even cut off by copper thieves.