The G train is one of the only lines, along with the C and E, that along its length is entirely in the Independent system, the portion of the subway built by the city in the early 30s. It is almost entirely cut-and-cover construction, except for the portion under Newtown creek separating Brooklyn and Queens, and the brief elevated portion over the Gawanus Canal.

It was built as the 'Crosstown line' originally from the Queens Boulevard line and along its present route through Long Island City, Greenpoint, Williamsburg, Northern Brooklyn, downtown Brooklyn, Caroll Gardens, over the Gawanus Canal, through Park Slope, and on to Church Avenue. The portion from downtown Brooklyn to Church avenue later became and is currently used primarily for Manhattan service through Jay street (the F train) but the G still runs part of the way and terminates and Smith/9th. The elevated portion over the Gawanus Canal is the only elevated porion of the IND as originally built, and Smith/9th is the highest station on the system, offering a magnificent view of downtown Brooklyn and Manhattan.

The G currently runs exclusively R44-46 rolling stock, a car order from the early 70s of tastefully appointed 75' cars. It is currently the only line that runs short trains at most times. There are plans to scale back service on the Queens end to Court Square and end the Queens Boulevard portion of the service.