The MTA on Monday revealed details of its hotly anticipated five-year capital plan — with a $51.5 billion price tag, more than $40 billion of which will go to fixing the city’s decrepit subway and bus systems.

Much of that cash will be used to fund transit chief Andy Byford’s “Fast Forward” plan, which includes modernizing antiquated signals on six lines within five years, officials said.

In total, 11 routes stand to benefit from the $7.1 billion signal modernization effort — the A and C on the Fulton Street Line, the 4, 5 and 6 on the Lexington Line, the N and W in Astoria, as well as segments of the G, F, M and E trains.

At the end of the capital construction process, over 50 percent of subway riders will “be served” by lines with modern signals, the MTA said.

The capital plan also proposes $6.1 billion for new subway cars, $2.6 billion for track maintenance, $5.2 billion for station accessibility, $4.1 billion for other station improvements and $2.9 billion for the second phase of the Second Avenue Subway.

The agency also wants to spend $2.5 billion on 2,375 new city buses, including 500 electric buses at $2.2 million a pop.

But it could be in for a fight with Mayor Bill de Blasio — as it is asking the city to chip in $3 billion of the total cost, as much as the state’s anticipated contribution.

De Blasio and Gov. Andrew Cuomo spent months in heated negotiations over their respective contributions to the last capital plan, which cost $32 billion. The state ultimately contributed $8.3 billion to the city’s $2.5 billion.

A similar spat played out in 2017 over Cuomo’s $835 million emergency fix-up to the subways during 2017’s “Summer of Hell.” De Blasio eventually agreed to pay half.

A rep for Hizzoner said de Blasio is “reviewing” the new capital plan.

But agency officials argue the 50-50 split is “appropriate” this time around given the massive amount of funds going to city-based subways and buses.

“The $40 billion number dedicated to New York City Transit — subways, buses, across the system — exceeds the entire existing capital plan, that’s extraordinary,” MTA chairman Pat Foye said at a press briefing Monday.

The agency is already counting on $25 billion to be funded by a new internet sales tax, new tolls on city-owned bridges, and a new tax on homes worth over $2 million.

Another $9.8 billion will be covered by borrowed debt — to be paid back, ultimately, by city straphangers’ fares and tolls.

“This plan exceeds my wildest expectations,” Byford told reporters Monday. “We are going to make sure that we deliver on this capital plan and make sure we get this system resignaled in an unprecedented amount of time.”

The MTA’s board will vote to approve the plan by Oct. 1, officials said. After that, it requires unanimous approval from the Capital Program Review Board, whose four members include representatives of the governor, mayor and each chamber of the state Legislature.

Transit advocates have derided the agency for what they view as a lack of transparency in the plan’s development, roll-out and approval.

“The board hasn’t seen the capital plan and there’s one week to vote,” said Lisa Daglian of the Permanent Citizens Advisory Committee, the MTA’s in-house advocacy group. “For any [board] members that think that’s adequate time to review a $51 billion plan, we beg to differ.”

Additional reporting by Julia Marsh