Where to begin? For one thing, there’s the old—so very, very old—infrastructure. “In fact it’s so old that the MTA can no longer buy replacement parts from the manufacturer,” James Somers wrote in a 2015 essay for The Atlantic. “It has to refurbish them itself.”

The 1960s-era Brightliners, those stainless-steel C-train cars, break down constantly—every 33,000 miles on average, The New York Times recently reported. That’s compared with the average subway car, which breaks down every 400,000 miles, and the newest cars, which break down every 750,000 miles, according to the newspaper. Then there’s the signaling system that Somers wrote about. On top of being ancient and unreliable, signals are inspected far less frequently than they were a decade ago. They’re languishing despite sorely needed upgrades that could otherwise improve efficiency to accommodate the growing throngs of riders.

Oh yeah, the people. Here’s where we get into tipping-point territory that explains how things seemed to get so bad so quickly. There are more passengers now than there have been since the 1940s. The all-time ridership record was set in 1946, the year 2 billion passengers rode the subway. Ridership exceeded 1.7 billion last year, and broke records set in 1948. These days, overcrowding is the reason for about one-third of the system’s delays any given month, the MTA says. (Then there’s Penn Station, which is facing its own chronic failures after “decades of neglect,” as New York Governor Andrew Cuomo put it in May.)

“The other factor is there’s no political capital in doing preventative maintenance,” says Andrew Natsios, the former chairman of the Massachusetts Turnpike Authority who helped save Boston’s notorious Big Dig highway project. (He’s now the director of the Scowcroft Institute of International Affairs at Texas A&M University.) “The consequence is that systems deteriorate much faster. No one gets elected for voting for preventative maintenance. It’s very sad.”

Even when state officials have set aside the necessary money for upgrades, funds haven’t been managed well, and repairs are rarely made on schedule. Cost overruns are a big issue on any major infrastructure project, but especially when you’re dealing with an old urban area. At least New York officials now seem to be taking the subway’s many problems seriously. Lhota’s return to the authority and a recently announced modernization competition are reasons for cautious optimism. Then again, “there are always problems you don’t expect,” Natsios told me.

He gives an example from his time in Boston, when Big Dig construction disturbed the city’s many rats—and the rats disturbed the city’s many humans. “The rat population under Boston goes back to the Colonial period,” he said. “You suddenly had tens of thousands of rats on Boston streets. They had to spend tens of millions of dollars on rat extermination.”