The MTA has suspended its Grand Central Terminal guru for allegedly giving secret, forbidden tours of the commuter cathedral, sources said.

Dan Brucker worked for the MTA for three decades, most recently guiding tourists through “off-limits” areas at Grand Central, including a power station, a control room and secret rails that carried former President Franklin Delano Roosevelt’s limo underneath The Waldorf-Astoria hotel.

But he’s also suspected of giving the same behind-the-scenes tours on his days off, and without the MTA’s blessing — a serious security risk, one agency insider told The Post.

“There’s a reason why these places are off limits — for security,” the source said. “You don’t want people knowing about certain stairways and where they lead to. What if he’s unknowingly taking a member of ISIS?”

MTA-sanctioned tours are run by the guide group Orpheo Inc.

Brucker, who made $102,547 in 2015 as an MTA employee, was Orpheo’s head guide until about five weeks ago, when he “retired,” according to workers there.

“He retired a couple weeks ago,” said a man answering phones at Orpheo’s Grand Central Terminal outpost. “There are no behind-the-scenes tours available right now.”

Brucker was the go-to guy for intimate looks at the terminal, and Orpheo advertised Brucker-led tours of the “off-limits” areas before his retirement.

The MTA confirmed it is investigating Brucker, but did not say whether he was allowed to take people into off-limits areas on agency-sanctioned tours. Doing so during unsanctioned tours is not allowed, the MTA source said.

Brucker was one to bend the rules, according to Long Island photographer Roni Chastain, who has gone on two of his expeditions below the terminal.

Chastain and some fellow photographers showed up at Grand Central one morning in March 2014, hoping to get Brucker’s behind-the-scenes look. Brucker’s day was booked, but he cut a deal with Chastain and crew to take them up to the terminal’s glass catwalks for a few minutes so they could take pictures — though he did not charge for the access, according to Chastain.

“He said, ‘Meet me by the clock at 10 a.m., and I will take you up to a glass walkway,’ ” Chastain told The Post. “It was very fast, because he was doing another tour. He did it out of kindness of his heart.”

Brucker did not return calls.