The Metropolitan Transportation Authority launched the biggest schedule shakeup in 50 years at the Long Island Rail Road on Monday — with dozens of trains serving its new Grand Central Madison terminal, in part by slashing service between the suburbs and Brooklyn.

Crowds spilled across the platforms at the LIRR’s sprawling Jamaica hub as riders navigated the dramatic changes, which also forced the railroad to shorten its trains so it could free up cars for new added service to the gleaming new Midtown East hub.

“BK customers really left in the dust,” texted one LIRR rider who asked their name not be used but sent along a photo of a packed platform at Jamaica. “It was more chaotic than usual. Much more crowding. Not a faster commute.”

Another rider said she didn’t have a choice other than to find a way to make the new service work — though she offered compliments for the MTA’s recently overhauled commuter railroad app.

“I just look at the app. I have to get where I need to go regardless,” said Tamara Dorismond, a 27-year-old commuter. “It’s just a new schedule, we don’t really have a choice.”

Meanwhile, some riders making their first trip into the gargantuan $11 billion LIRR train hall beneath Grand Central complained that poor signage made it difficult to navigate and that it takes so long to get down to the platforms — which are 150 feet deep — that much of the time savings from getting dropped off in Midtown was eaten up.

“It took me 10 minutes to figure out how to get down here. I could’ve walked to Penn Station in the same time with how long it took me to get down here,” said commuter Prudence.

“The schedule is pretty bad too. I’m sure it will get better —- (the station) is very pretty though.”

Others were more positive.

“This makes my commute about an hour less,” said 21-year-old Nick Jablonski. “Comparing the 20 minute time to get here versus the 45 minute time to Penn Station, it can actually be different if I do this a lot.”

Officials said that roughly 30 percent of the Monday morning rush hour to Manhattan opted to head to Grand Central Madison.

MTA officials have touted the new schedules and claim they increase service on the LIRR by 40 percent when compared to the amount service the railroad offered in 2022, a baseline that is well below the number of trains run before the pandemic.

All told, the MTA expected to run nearly 1,000 trains on the LIRR on Monday — almost 300 were bound or departing from Grand Central Madison.

The new Midtown hub gives the MTA the ability to dramatically expand LIRR service from Long Island to Manhattan, which had been constrained by the lack of space at Penn Station before the coronavirus pandemic hit and ridership plummeted.

Additionally, officials say the extension of a third track on the Main Line to Hicksville will allow the MTA to provide consistent reverse commute service for the first time between the five boroughs and some of its biggest suburbs. 

Officials previewed the schedule changes in the fall, but LIRR rider activists have assailed the railroad for not making major adjustments to the drafts before rolling them out on Monday.

The new gains come with some major trade-offs for the LIRR and its riders.

MTA officials scratched much of the schedule running into Atlantic Terminal to free up trains for their new midtown hub — replacing it with a new shuttle that runs every 12 minutes at peak and every 20 minutes during the rest of the day.

The shuttles aren’t timed against the rest of the LIRR’s schedule at Jamaica, a decision that left many cold and frustrated as they raced to make the connection or saw it pull away in the distance.

That threw a wrench into long-established commutes for Long Islanders headed to Brooklyn or the Financial District, who would have used Atlantic Terminal as an easy transfer point to the subways.

“It’s on a new track. I didn’t even realize until just now. Now I’m rushing to get to the next train,” said one man, racing to make the transfer.

Passengers heading to Lower Manhattan, the Financial District and Brooklyn accounted for 28 percent of the LIRR’s ridership, according to a 2016 study. That’s a larger percentage than those heading to Midtown East or the Upper East Side.

“My [second] attempt at a Brooklyn bound train led me sprinting stairs again with a mob of commuters,” said Patrick, messaging a Post reporter. “This train was late by 10 min.”

The MTA did not immediately respond to a request for comment.