LIRR commuters have been plagued by overcrowding and botched schedules for months — because no new trains were ever ordered as promised for Grand Central Terminal, documents show.

The feds began warning the Long Island Rail Road as early as July 2017 that it was falling behind schedule to order and receive the roughly 20 eight-car trains it needed to run the promised schedules at its new $11 billion terminal beneath Grand Central, according to reports from the Federal Transit Authority obtained by The Post.

And the order for the train cars remains pending to this day.

LIRR officials eventually told the FTA in 2020 that they would find the trains from “the existing LIRR fleet” — which meant taking trains that already served Penn Station or Brooklyn’s Atlantic Terminal and moving them to the new Grand Central Madison site.

The move cut capacity into Penn Station, required commuters to make new transfers at Jamaica when going to Brooklyn and ended timed connections.

“It’s my opinion that they overreached with the schedule when they opened Grand Central Madison,” said Gerry Bringmann, the LIRR’s rider representative on the MTA Board of Directors.

“They overreached on the schedule because we did not have the rolling stock to accommodate it,” he said. “More trains would help.”

Infuriated riders, who pay as much as $30 each way to commute into the city, have bitterly complained about the ensuing issues.

The LIRR has since reversed a few of the changes and restored limited direct service to Brooklyn.

But taking trains from the existing fleet contradicted years of promises that Grand Central Madison’s opening would dramatically increase service between Long Island and Manhattan.

Officials wrote in the project’s environmental review that Penn would keep its schedule of 37 peak-hour trains while adding another 24 by way of Grand Central.

A May analysis by longtime LIRR commuter activist Patrick O’Hara revealed the railroad is running just 37 trains per hour to both stations, meaning that frequencies to Penn Station were slashed.

The warnings from the FTA’s Project Management Oversight Committee began as early as July 2017, the records show.

“The [federal watchdog] is concerned about the continued schedule slippage of the LIRR vehicle procurement program because it has the potential to severely impact delivery of the vehicles, and, hence, [start of service],” the report from that month states.

The LIRR finally began the process of requesting bids for 160 new train cars, known as M9s, in November 2017, a process that the FTA’s encouraged the railroad to wrap up as “quickly as possible.”

The watchdogs again flagged the stalled M-9 contract in July 2018, which the LIRR then promised it would award that December.

By April 2019, that deadline had slipped to June 2019. The railroad blew that deadline, too, and admitted to the federal watchdog in July 2019 that the trains would not be ordered in time.

“If that is the case, MTA will need to determine how to supply vehicles from its existing fleet in order to begin LIRR service to Grand Central Terminal,” the watchdog warned in that report.

The railroad acknowledged that was its plan in August 2020.

“The MTA is actively engaged in negotiations for the procurement of M9A cars,” said MTA spokeswoman Joana Flores to the Post. “This procurement has been significantly delayed predominantly due to changes in market conditions and the direct and residual impacts of the COVID-19 pandemic.”