Trailers.
BU gate car 1273 sits with locomotive 884 at the South Brooklyn Marine Terminal. The next day, the gate car would be loaded onto a trailer to be a float in the "Hometown Heroes" ticker tape parade on 7/7/2021.
1273 is the oldest rapid transit car still on NYCTA property, having been built in 1903 for the Brooklyn Union Elevated Railroad (BUERR or BU). It is also one of few trailer cars of this type that still exist, having no propulsion equipment of its own. Fitting that it will be placed on a (truck) trailer for the parade!
This part of Brooklyn is rich with railroad history. In the background is Industry City, which started life as factory lofts owned by the Bush Terminal Company, a major rail-marine contract terminal that floated railcars across New York Harbor. The trackage used to access NYCTA property from here is part of the South Brooklyn Railway, which formerly ran freight along the BMT West End Line to 9 Av (today's D train), then via 37th St and McDonald Ave to Coney Island Yard, serving industries along the way. Per the work train manifest entry for this job, SBK locomotive N1 was requested if available, but unfortunately it seems neither it nor sister N2 were.