In quite a few cases, old rail right-of-ways near cities are large enough for an extra track or few. Because, back in the heyday of American railroads, they either had another track or few, or they expected to.
The biggest issue is often bridges. Retaining the land that additional track(s) were on is fairly cheap. Building and maintaining rail bridges is not.
And building the light rail bridges for a transit system is not cheap. It's just less horribly expensive than building bridges which you could run strings of 220-ton freight locomotives over.
Funny reason there used to be double tracks almost everywhere that is now single tracked: while the government granted the property to the railroads, they still excised a tax over the portion of that land used by the railroads, so in the 70s when companies were going bankrupt left and right they tore up their own infrastructure to reduce the tax burden. Hell of a fuckup.
To really be usable - by revenue-generating trains - track has to receive regular maintenance. Which costs money. If your RR is desperately short on both revenue-generating trains and money, then it's kinda obvious that you cut the no-longer-necessary expenses.
And railroad rails are steel, generally weighing 100+ pounds per yard. Scrap steel sold for far fewer dollars per ton in the '70's - but you get about 200 tons per mile of unused track that you tear up.
The biggest issue is often bridges. Retaining the land that additional track(s) were on is fairly cheap. Building and maintaining rail bridges is not.
And building the light rail bridges for a transit system is not cheap. It's just less horribly expensive than building bridges which you could run strings of 220-ton freight locomotives over.