“1-3-2,” he said on Twitter. “4 and 5 don’t exist when you’re 6’6’.”

Michael R. Bloomberg, Mr. de Blasio’s predecessor and a regular subway rider, agreed with Mr. Byford that standing was the best option.

“I always stand,” Mr. Bloomberg posted on Twitter, alongside a photo of him reading a newspaper on the train, although he was committing his own etiquette lapse by blocking the door.

Some New Yorkers wondered how a train could possibly be empty, as the one in Mr. Bautista’s photo is, and whether it was also delayed. (The criticism is fair: Subway trains are late about 20 percent of the time.)

Other riders had more practical concerns.

“Whichever one isn’t inexplicably wet,” the comedian Mike Drucker wrote.

One rider complained that the seat closest to the door could inspire a robbery. “I call that one the snatch and run,” he said of seat No. 1.

Mr. Bautista, 20, said his train was empty on New Year’s Eve because he gets on at the first D stop in the Bronx. He was home for the holidays from Marist College in Poughkeepsie, N.Y.

“I didn’t expect it to blow up the way it did,” he said. “Everyone has their own opinion. I thought everyone was on the same page.”