It happened north of the Hoyt-Schermerhorn station. Thankfully there were no serious injuries. From the MTA:
At 10:34 p.m, the front axle (first two wheels) of the head car of a Church Av-bound G train derailed 700 feet north of the Hoyt-Schermerhorn Sts station. Approximately 80 customers were safely walked through the 4-car train and the tunnel to the Hoyt-Schermerhorn Sts station. FDNY reports that one customer complained of dizziness and two others were being evaluated at an area hospital for minor injuries.” The agency is also investigating the cause of the derailment.
Here’s a firsthand account via Gothamist:
Some flustered MTA workers boarded the train, but refused to answer any questions about what happened. It was pretty clear we were off the rails – the wall was disconcertingly close to the right side windows. People started to get sweaty and anxious, but mainly the atmosphere was chill.
A woman with a Chipotle burrito-sized newborn baby was understandably upset, especially as it got hotter.
After about 15 minutes, the FDNY arrived with fire-axes ablaze and they seemed to have plan. They crowded it us all into the first two cars and a fireman who was clearly very experienced in crowd control calmly explained: The train is derailed. We are waiting for third rail power to be shut off, after which we would walk out the front doors of the train in an orderly fashion, on to the catwalk, and proceed down the tunnel to the Hoyt station.
The tunnel was incredibly grimy and it smelled like grandma’s basement. The track bed was already filled with workers, guiding passengers by flashlight and investigating the train. After a 5 minute hike, we were greeted by hundreds of FDNY, paramedics, and MTA workers at the Hoyt platform, all of whom seemed relieved that we were all in one piece.