It’s the Wild West down there.

Major crime spiked on city subways last week — even though the platforms practically have tumbleweeds amid a severe drop in ridership — as arrests and summonses were nearly nonexistent, data shows.

The NYPD recorded a more-than 35 percent spike in felonies between April 20 and Sunday, with 38 such crimes reported over the span compared with 28 over the same period last year, according to department data.

The rise was largely driven by a jump in robberies, which went up from three to 13 over the span.

“Some crimes are up in the subways, even though ridership is down 90 percent,” Gov. Andrew Cuomo said at his daily press briefing Wednesday in Albany. “I don’t even know, mathematically, how that is possible.”

Over the last month, ridership averaged less than 9 percent of normal tallies, MTA data shows. The rise in crime bucked the trend over the last four weeks in which the transit system has seen such incidents drop by nearly a third.

Arrests, meanwhile, have plummeted over the last month, declining by nearly 87 percent — from 955 to just 124 — while cops issued fewer than 300 summonses compared with more than 13,000 over the same time last year. Arrests were down 83.8 percent for the week, too.

Much of the broader, monthlong decrease in crime came thanks to a sizable dip in grand larcenies, with straphangers only reporting 44 such felonies over the last 28 days compared with 105 in the same period last year.

Felony assaults also dropped by 10 complaints — from 30 to 20 — in the same span, the data shows.

But robbery and burglary have both been on the rise, according to the NYPD numbers.

Rail-riding wrongdoers, however, will now encounter an enhanced police presence in some stretches of the system — but it has nothing to do with the uptick in crime.

The MTA on Wednesday welcomed Mayor Bill de Blasio’s offer to deploy NYPD cops on overnight runs to help clear the system’s burgeoning homeless population off of trains — an effort to protect both the homeless New Yorkers and essential workers commuting to work each day.

“It is making a world of difference already for our customers and employees,” said Sarah Feinberg, interim president of New York City Transit, in a briefing.

The effort began Monday night when approximately 40 cops, 10 social-service workers and a half-dozen MTA police officers ushered some 100 homeless from lower Manhattan’s WTC Cortlandt station’s No. 1 line. Then, late Tuesday into early Wednesday, a group swept the 96th Street station at the Q line’s northern terminus to similar success, according to Feinberg, who said it was one of 10 stations they hit.

“We need them to be in all 41 end-of-line stations, which is where we have the biggest issues,” she said.

On Monday, Hizzoner had offered to send cops to 10 end-of-line stations if the MTA agreed to shut them from midnight to 5 a.m. — to which Feinberg had later replied that she wanted cops in every terminus.

Hizzoner pressed the MTA to take “yes” for an answer, in a City Hall press briefing held prior to Feinberg’s Wednesday remarks.

“To be game changers, we have to disrupt business as usual,” said de Blasio.

“It will change the pattern that has existed for decades, where homeless could ride subway lines back and forth and back and forth,” he continued. “We’re kidding ourselves if we think we can get a different result by doing the same thing over and over again.”

Police sources said that officers were ready to make the sweeps a nightly occurrence near 10 targeted terminals.

Each train nearing the end of its run between midnight and 5 a.m. will be emptied of all riders at the lines’ penultimate stations, sources said. Shuttle-bus service will be provided for those travelers continuing on to the last stop.

Feinberg signaled that she not only embraced de Blasio’s proposal for the short term but wanted to make it the norm even in a post-coronavirus world.

“This needs to be a long-term commitment, not just something that [will] get us through the most recent cold spell or until the weather breaks, or until we can get through this hump,” she said.

Additional reporting by Tina Moore, Larry Celona and Bernadette Hogan