A man uses an e-cigarette in this file photo. (Reuters/Christian Hartmann)

A transit rider was spotted on a Red Line Metro train about 10 a.m. Monday morning sitting alone, engulfed in a cloud of smoke.

Barry Jackson, the Metro rider who snapped a photo of the conspicuous smoker around the White Flint station in Maryland, said in an e-mail the young man appeared to be smoking a tobacco substance, perhaps clove cigarettes.

Smoking cigarettes on Metro is prohibited. But what about e-cigarettes? That’s a little more ambiguous. (Smoking marijuana, of course, is also illegal on Metro out of any device.)

Back in May 2013, Post columnist John Kelly saw someone vaping on Metro and asked that same question. Metro spokesman Dan Stessel said at the time that it’s local ordinances that determine what’s legal and not on Metro. (Which explains why you can legally possess marijuana at a D.C. Metro stop, but not one in Virginia.)

“I think it’s unclear,” Stessel said at the time. “I think this is a conversation that the legislative bodies are just beginning to consider. I think it will be something that there’s increasing clarity around as we move forward in time.” Stessel said Monday that this still applies.

In 2010, Virginia ruled that the state’s smoking ban does not apply to e-cigarettes. D.C. tried to pass e-cigarette restrictions, but that measure ultimately failed. Maryland is mulling legislation to apply cigarette restrictions to e-cigarettes.

According to Barry, the smoke around the impolite Metro rider was thick. None of the passengers said anything to him, but those sitting near him seemed “put off.”

“At the first stop, he puffed the first time and I made a point to look and keep eye contact,” Barry wrote in an e-mail.  “Kind of like — a disapproving gaze.” Barry tweeted a photo of the incident, which has garnered attention on social media.

So here’s the takeaway, D.C. transit riders: Definitely don’t smoke marijuana out of any device on the Metro. Don’t smoke cigarettes. And smoking an e-cigarette on the Metro is bad etiquette, but, well, it won’t land you behind bars.