Old skeleton of tracks at Alabama Ave on the J

At 19:15 of this video there is what looks like former trackwork above the station, what used to run there?

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Teh structure that you are referring to was a provision for an express track on the Jamaica Avenue Line, which presumably would have been double-decked from Alabama Avenue to at least Crescent. The track structure that you see ends just west of Alabama Avenue Station. It was never used.

Larry, RedbirdR33

The "Alabama Avenue Launch Pad"

The elevated structure between Alabama and Crescent is original to the 1880s, it was found to not be able to take the weight of a third track.

I've noticed that the Jamaica line, the elevated part is for it's entire length wide enough for 3 tracks - but while there are some short stretches with a center track, most of the line has a wide open space between the 2 main tracks, some of it with open trackway, or catwalk planking, maintenance shedding, or a station. Why bother to make the structure that wide without oactually having a third track?

When the BMT built the jamaica line, they saw no need for a 3rd track and the expenses that come with it (capital and operational) but just to make sure that they could add one if need be, they built the line with a provision for one.
The only lengthy section of 3rd track is from Marcy to Broadway junction. From Marcy to Myrtle, it is in regular service, and from Myrtle to Broadway, deadhead moves use it to and from ENY yard.
As a side note, the jamaica line actually has express service but in the form of rush hour skip stop service between Myrtle and Parsons boulevard.