You can't blame the MTA for no air conditioning on the subway cars if they're simply trying to deliver you an authentic experience! The agency brings the Nostalgia Train back tomorrow morning, inviting you to join them "on the ever popular boardwalk-bound Coney Island Nostalgia Train, riding the R1-9 cars of the Independent Subway System... it’s 'All Aboard' for a scenic (above and below ground) two-hour ride to Coney Island."
The trains don't have air conditioning (only ceiling paddle fans), and the high is supposed to be around 98° tomorrow—so at least bring your bathing suit so you can jump into the ocean once you deboard. And here are some ways you can keep cool underground.
sauna historic experience will cost you $50 if you're an adult, and $20 for children (you will also get exclusive access to the exhibits and store at the Transit Museum)—and there's another one coming up in August that'll bring you to the Rockaways.