Sen. Bernie Sanders took a subway ride with Mayor Bill de Blasio on Monday — but only after waiting 10 minutes for a downtown A train.

The progressive bosom buddies met at Penn Station shortly before 11 a.m. to ride just five stops to Fulton Street, but were forced to make small talk on the platform because of delays in service.

As they waited, they were serenaded by a pair of musicians, and then caused a ruckus when they entered a packed train with dozens of reporters, photographers and bodyguards in tow.

Riders shouted out encouragement, snapped photos and shot videos — and at one point, a man started shouting, “Lock up Trump! Lock up Trump!”

Sanders suggested Hizzoner’s so-called millionaires tax proposal to fund the MTA long-term could work miracles.

“When you ride on the subway, you should ride in comfort, you should ride in a subway car where you can sit, where you know where the train is coming,” said Sanders. “That is not a radical idea. It exists around the world.”

The proposed tax faces a rough road in Albany, which would have to approve it.

Sanders insisted he wasn’t taking sides in a battle between Gov. Andrew Cuomo and de Blasio over how to fund the subways — even though Cuomo has called the millionaires tax proposal “dead on arrival.”

“I’m not taking sides — to be honest, I’ve got enough to keep me busy in Washington without getting involved in New York state politics,” Sanders said. “[To] ask people on top to pay a little more in taxes, I think that is fair, I think it’s the right thing to do.”

The mayor — who endorsed Hillary Clinton in 2016 over Sanders — echoed Sanders’ message of a rigged financial system that has propelled those on Wall Street to the top of the heap.

“They got a lot of what they got because of laws that benefited them specifically, and tax laws written for them,” said de Blasio.

The mayor for months has declined to support calls for half-fare Metrocards for the lowest-income New Yorkers — despite a hefty city budget surplus — and only embraced the idea as part of his proposed tax on the wealthy.