He’s going off the rails.

A day after nuking the MTA’s ­L-train shutdown plan, Gov. Andrew Cuomo on Friday again blindsided the agency when he proposed scrapping its planned 10-year upgrade of its aging signal system in favor of “Tesla-like technology.”

Cuomo said he’d already been in touch with CEO Elon Musk’s electric-car company about the gadgetry — accusing the MTA, which the governor controls, of being captive to a “transportation industrial complex” run by risk-averse consultants and contractors.

“I called Tesla because it’s outside the box,” Cuomo said during a morning conference call with reporters.

“I don’t believe you need seven, eight, 10 years to put in a new [signal] system . . . at a time where they’re talking about flying cars, and you can get into a car and drive 100 miles on the [Long Island Expressway] and never touch the steering wheel, that there’s not a better technology for subway trains.”

But hours after his proclamation, Cuomo backtracked.

He released a statement saying the MTA will continue with the current plan “until an alternative — such as [ultra-wideband]” is “determined to be viable.”

That plan is called Fast Forward — which New York City Transit chief Andy Byford spent much of the past year promoting. It comes with a $37 billion pricetag and will replace the subway’s failing signals with newer technology over the next decade.

However, Cuomo said he was dazzled by the promise of a signal technology called ultra-wideband radio that he thinks would be better and faster to install.

“It’s not something the vendors in the MTA use,” he sniffed, accusing the agency of being “captive” to its suppliers.

“I understand [Tesla] is not a [typical] subway vendor,” Cuomo added. “So what?!”

The newfangled tech would allow the agency to track where trains are at any given time and increase service — but it has yet to be proven viable in the city’s labyrinthine subway system.

Cuomo also noted that the MTA board will still have to approve the L-train fix that he made sound like a foregone conclusion on Thursday — when he waved off the long-planned shutdown of the line’s crumbling tunnel for his own scheme to perform a speedy patch-up job after-hours.

When asked Thursday if the board would need to sign off on the revised plan, Cuomo said no — before being corrected by its acting chairman.

The board members themselves only heard about Cuomo’s new L-train scheme that day.

“The board was not told. The board was not involved, there was a flurry of emails about this yesterday,” said MTA board member Andrew Albert.

“Will anyone want to do a contract with the MTA anymore if it can just be abrogated like this?”

Experts and officials were fuming at the governor for throwing his own agency’s plan under the bus again Friday, saying he is being irresponsible by announcing these ideas to the public instead of working through and testing them with MTA engineers.

“If this works, terrific, but this is not a way to work in collaboration with people who spend a tremendous amount of time trying to operate a system,” former MTA board member Allen Cappelli told The Post.

“The governor has the most say about [the MTA] — that he would choose to not engage with this project and at the 11th hour try to upstage and try to become a hero is not a good way to govern.”

The MTA has good reason to be wary of rushing head-first into untested technology, said a former high-ranking agency official.

“The engineers at the MTA can have tunnel vision, but they are cautious for a reason: Plenty of big ideas blow up. If it blows up at the MTA, hundreds of thousands of people will suffer,” said the ex-official.

“People at the MTA use caution on big new ideas because they’ve seen big new ideas fail in the past . . . [Cuomo], of course, does not have to accept the recommendations of the smart people doing this their entire career.”

Cuomo’s rogue actions are also a kick in the teeth to Byford — who was recruited in late 2017 while the subway system was melting down — said one good-government expert.

“Byford is one of the top transit experts in the world and he’s expressed skepticism because it hasn’t worked anywhere in the world,” said John Kaehny of Reinvent Albany.

“The agency is supposed to be run by transit professionals who rely on other transit professionals. For the governor to order the MTA to research this speculative technology puts a lie to idea that the MTA is independent or that the MTA board can even perform its legal duties.”

The Post repeatedly reached out to Byford for comment, but the MTA and the governor’s office refused to make him available.

Additional reporting by Ruth Brown