When the weather outside was frightful, a commute on the 7 train was awful.

That's because new equipment installed as part of a $600 million overhaul of the line's signal system malfunctioned after a light dusting of snow.

It made the MTA's transit chief, Andy Byford, furious. "They know I'm on the warpath about this," he said.

The malfunction forced the MTA to cut top train speeds nearly in half, to 25 miles per hour, during Wednesday morning's rush.

"Everything has their kinks or things they got to work out. Hopefully they'll learn from it and go from there, make it a little bit better," said commuter Raul Morales.

Byford says he dressed down a top official with the French company that made and installed the equipment, Thales.

"I told them they're putting in jeopardy their prospects for future work at NYC Transit," said Byford.

The new computerized signal system enables trains to run trains closer together, increasing capacity and reducing overcrowding and delays.

The covering of snow affected the transponders that allow the system to pinpoint the location of each train.

Thales has known about the weather-sensitive equipment since an outage in March, but had not completed installing protective covers on all the transponders.

Byford sent the company a letter blasting its "inexcusable failure" and complaining it’s “remedy has been too slow, inadequate in its scope and unacceptably ineffective.”

"If they don't fix it, there will be hell to pay," Byford said.

A similar signal system installed on the L line by a rival company has operated flawlessly.

In a statement, Thales said, “All our teams are fully resourced, mobilized and committed to deliver and deploy the solution as quickly as possible in order to offer New York commuters the best level of service and avoid speed reductions in such weather conditions.” .

The 7 line operates between Flushing, Queens and Hudson Yards in Manhattan, carrying up to 60,000 riders an hour during peak commuting times. Installing the new signals caused years of service shutdowns and delays. The work was completed last fall, two years behind schedule.

The MTA said Thales will put covers on all the affected equipment and fix its software so the system operates as it should.