Many visitors to the DUMBO, Brooklyn area mistake the numerous tracks found in the Belgian-blocked streets for old trolley tracks. However, since until a few years ago DUMBO was almost entirely given over to warehousing and manufacturing (except for the small Vinegar Hill neighborhood on the eastern end) trolley lines never troubled it north of Sands Street.
One clue that these were never trolley lines is that several pairs run on sidewalks and into buildings. These tracks belonged to the Jay Street Connecting Railroad, one of the shortest rail lines in Brooklyn. It was originally constructed (beginning in 1904) by the Arbuckle Brothers, who imported coffee from far-flung regions; it was unloaded from carfloats in the East River (not for nothing are there two streets in DUMBO called Dock and New Dock). Goods were shuttled into warehouses a lofts in the streets of DUMBO via the tracks. The JSCR ran its last load in 1959.
The whole story:
William Bryk in New York Press
JSCR on Trainweb