Dyre Av. historical Question.

There wasn't any reason to do that in the 1940s because the Pelham IRT (now the #6) pretty much served the same area.

But the Second Avenue subway plan of the 1970s would have used that route; I think there were plans to redevelop the Harlem River Yards (as housing?), and a new station on the subway would have served that. The northern end of the line would have used the old NYW&B station at East 180th Street. The point of that was so riders from the northeast Bronx could transfer to a fast bypass route into Manhattan.

About FRA compliance -- Did that apply before NYC took it over, and is it now?

the FRA didn't exist until the passage of Department of Transportation Act of 1966 (49 U.S.C. § 103, section 3(e)(1)). the line had been part of the subway for about 15 years by that point.

In any case, a right-of-way or even a particular track can be, I think, transferred from an FRA-regulated railroad to a non-FRA transit operation.

In Bayonne, for example, the freight track on the east side of the ROW is FRA-regulated but the two Hudson-Bergen light rail tracks on the west side are not regulated. There is a fence with warning indicators (in case of a freight derailment) between the two.