NEW YORK—Hackers infiltrated computer systems for the Metropolitan Transportation Authority in New York, setting off a scramble to counter a potentially crippling cyberattack against North America’s largest transit system, MTA officials confirmed on Wednesday. The officials said in a statement that the agency received an alert from the FBI and other federal agencies saying three of its 18 computer systems were put at risk. The MTA insisted that it quickly shut down the attack. It said a follow-up forensic analysis also found that no sensitive information was stolen and that rail service for millions of riders each day and other operations were never compromised or disrupted. “Importantly, the MTA’s existing multi-layered security systems worked as designed, preventing spread of the attack and we continue to strengthen these comprehensive systems and remain vigilant as cyberattacks are a growing global threat,” said Rafail Portnoy, the MTA’s chief technology officer. The cyberattack was …