NEW YORK CITY— More than half of the required maintenance checks at a Brooklyn subway electrical substation went unfinished before a transformer exploded in December 2024, stranding more than 4,000 passengers underground, according to an audit released by the Metropolitan Transportation Authority's inspector general.
The audit found broader gaps across New York City Transit's network of more than 200 substations, which convert high-voltage electricity into power for subway trains.
Nearly one in five required maintenance assignments went unfinished systemwide in 2024.
Workers also failed to address fire alarm trouble signals at every substation the inspector general visited with fire alarm systems, and electrical maintainers did not wear equipment rated to protect against arc flashes, according to the report.
At the State Street substation in Downtown Brooklyn, 12 of 23 required maintenance assignments, or 52 percent, went unfinished between April and the Dec. 11 explosion.
The blast sent flames through a sidewalk grate and stranded passengers between Jay Street-MetroTech and Hoyt-Schermerhorn stations for about two hours before first responders evacuated them.
No injuries were reported.
The transformer failed after copper tubing inside a heat exchanger split and leaked water into the synthetic oil used to cool and insulate the equipment, according to New York City Transit's internal investigation.
As the water accumulated, it mixed with the oil and reached live electrical terminals, causing the explosion.
The agency never determined why the tubing split.
The problem may not have been isolated to State Street. Three transformers made by the same manufacturer had evidence of internal water leaks, according to the inspector general.
Two are scheduled for replacement under the Metropolitan Transportation Authority's 2025-2029 capital program. The third also needs to be replaced.
New York City Transit now tests the water content of synthetic oil in all water-cooled transformers twice a year.
The inspector general found 19 percent of required substation maintenance assignments went unfinished across the system in 2024.
The rate improved to 10 percent in 2025 after the inspector general shared its analysis with Power Group managers. Beginning in July 2025, the share of missed assignments fell to 5 percent or less each month.
But State Street continued to lag.
40 percent of scheduled maintenance assignments there went unfinished from April through November 2025.
The report found supervisors had access to a management report showing overdue work, but were not using it to identify missed assignments and redirect crews.
During the April-to-November period in 2024, the 10 crews with the highest rates of missed assignments failed to complete between 19 percent and 51 percent of their required work.
The inspector general warned that leaving equipment unchecked for months increases the risk of equipment failure.
Although the subway's electrical network has backup power paths, failures can shift heavier electrical loads onto neighboring substations.
The inspector general visited 16 substations and found additional problems involving fire protection and electrical safety.
Six of the 16 substations had fire alarm systems.
At all six, the fire alarm control panel displayed a "trouble" signal indicating a problem with the system. Substation maintainers did not report the warnings, according to the report.
No New York City Transit office, monitoring center or outside company had responsibility for watching those trouble signals.
The substations also lacked automatic fire suppression systems such as sprinklers. Their protection consisted largely of fire blankets and extinguishers.
Smoke sensors inside rectifiers, which can send alarms to the Power Control Center when smoke is detected, also were not being maintained or tested.
"Fire alarm systems are an early warning system designed to protect personnel and equipment," the inspector general wrote.
Inspectors found substation maintainers were not wearing personal protective equipment rated for arc flashes, sudden releases of electrical energy that can cause severe burns or death.
During site visits, employees described wearing regular hard hats, reflective vests, boots and rubber gloves. Some also had protective glasses. None wore the arc-flash-rated equipment required for the hazards inspectors observed.
Inspectors saw exposed copper conductors carrying high voltage, energized equipment with metal enclosures and damp or wet equipment rooms.
The report noted that much of the electrical equipment dates to the 1930s and lacks modern safety protections.
New York City Transit had allowed employees to purchase their own arc-flash protective equipment, but managers did not know the protection ratings of the equipment employees had obtained.
By late 2025, Power Group management had ordered the highest level of arc-flash protection for employees working on electrical equipment.
The inspector general also criticized New York City Transit's handling of the State Street transformer after the explosion.
Two days after the blast, inspectors observed the Power Group's Assistant Chief examining the damaged equipment.
The official had trained as a carpenter, not an electrician.
The inspector general's report said the official told investigators a transformer vendor or other subject-matter expert would be brought in to examine the equipment. No such assessment occurred.
The transformer also was disassembled during the initial inspection, preventing a later expert from determining what caused the copper tubing to split.
After the transformer was removed to make room for a replacement, the damaged equipment sat outside in poor weather, further compromising the possibility of determining the cause of the failure.
"The transformer never received the evaluation required to determine what led to the copper tubing splitting," the inspector general wrote.
New York City Transit had no formal protocol for investigating major substation incidents, according to the report.
The Substations Group oversees more than 200 facilities across the City. In 86 of them, some critical equipment is more than 50 years old and past its useful life.
Some substations still contain original equipment dating to 1932.
Replacement parts for some of the older machinery are no longer available from the original manufacturers. Workers have turned to third-party websites, retired equipment and parts fabricated in-house to keep some systems running.
The inspector general said older equipment also lacks modern safety features designed to protect workers.
The Metropolitan Transportation Authority's 2025-2029 capital program includes major improvements at more than 70 substations, including 21 full replacements, according to the report.
The agency is also developing a centralized system for tracking equipment defects, repairs and maintenance.
The audit made seven recommendations covering maintenance tracking, equipment records, defect monitoring, protective equipment and post-incident investigations.
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