The third Avenue El originally traveled all the way down 3rd Avenue to South Ferry. The line was slowly cut back: It was cut back to Chatham Square in 1950, and was cut back to 149th Street in 1955.

As far as tying it into another line goes, the only line with enough capacity would’ve been the Manhattan section, but since they tore it down, there was nothing it could connect to (it couldn’t connect to the Concourse Line even if they were able to build the connection because the line was built to IRT specs.

I do think they should’ve kept the portion in the Bronx, though I agree with them tearing down the Manhattan portion (even though it left the East side of Manhattan with an overcrowded subway line). I doubt the UES would’ve become the wealthy neighborhood it is today if the third Avenue El were still around (and, as people pay a premium to live in Manhattan, I think it would be wise to keep the property values as high as possible)

In the Bronx on the other hand, the El would benefit the area by providing transit service right to the neighborhood. even though it would be a blight, it would prevent the neighborhood from having to depend on buses for transportation.

They are a blight, but a necessary one to provide decent service to an area. for example, when I lived in Brighton Beach, the trains used to screech around that curve, but people put up with it because it provided fast, frequent service to Manhattan.