Metro’s new 7000-series trains will return to service after an electricity-related “hazardous condition” prompted the transit agency’s largest union to launch an emergency safety stand-down, causing hundreds of the railcars to be pulled from the system for the Thursday morning commute.

Amalgamated Transit Union Local 689 launched the urgent safety measure after learning that a mechanic had received an electrical shock last Saturday as he performed an inspection on a 7000-series railcar.

Rail cars must be inspected regularly before they go into service, and because workers with ATU Local 689 demanded a suspension of inspections on the some of the 7000-series rail cars in the system, Metro made do with fewer trains, arranged in shorter six-car configurations.

As a result, there were 18 percent fewer trains during Thursday’s morning. The unexpected shortage of cars resulted in significant delays and heavier-than-usual crowding for thousands of riders.

In a noon statement Thursday, Metro said engineers from the transit agency and from the railcar manufacturer Kawasaki had determined that there was no risk to putting the cars back into service, and that the inspection procedures for mechanics working on the trains were considered “appropriate and consistent with manufacturer guidelines.”

“Mechanical inspections of 7000-series railcars will resume this afternoon following additional safety briefings with employees to reinforce these procedures,” Metro said in the statement.

Metro spokesman Dan Stessel said there was no potential hazard to passengers, because there are secondary systems in place to prevent stray current from reaching other parts of the train.

“Customers can expect a relatively normal afternoon commute,” transit agency officials added in their statement, though “the proportion of six-car trains versus eight-car trains may be slightly higher than normal this afternoon as Metro works to resolve the inspection backlog and place more cars in service.”

Even so, riders still expressed frustration with the significant Thursday morning delays, particularly on the Red Line.

The “safety stand-down” followed an incident last Saturday in which a mechanic was shocked at a West Falls Church rail yard while he worked on a 7000-series car. According to an incident report obtained by The Washington Post, the mechanic indicated that the shock was “light” and left no visual burns, and he was taken to Inova Fairfax Hospital for evaluation.

After the incident, all maintenance work ceased on the train while the situation was assessed. According to the report, “it appears that car R7022 lost all grounding capabilities,” indicating that the car had electrical current running through it.

Stessel said that safety officials are conducting an investigation into the incident and have notified the Federal Transit Administration about a “hazardous condition.” After removing power to the car and suspending all other work, it appeared that the shock occurred due to “defective wires within the ground brush compartment,” he said.

“Metro has already issued a safety bulletin to all rail car maintenance employees, as well as the union, to advise them of the potential hazard and has implemented new procedures, effective immediately and until further notice, that require ground brushes on 7000-series rail cars be visually inspected before applying power to the vehicle,” Stessel said.

The “ground brushes” on a rail car are circuits that are attached to the rail car axles, which help return electric current from the train back to the rail. The components are located underneath the train cars and inaccessible to passengers.

In a statement Wednesday afternoon, the Amalgamated Transit Union Local 689 said the incident “could have ended in death or serious injury” and was one of several significant safety mishaps that have occurred to rail maintenance workers so far this year while servicing Metro’s new fleet of 7000-series trains, though it did not provide details on the nature of any of the other incidents.

“Metro knew of these electrical shocks since at least January, but covered it up and only decided to take action today because there was a victim involved who could have been killed,” the union said in its statement. “These actions are an abject failure to implement an effective safety culture when it should be Metro’s number one priority.”

Stessel said Metro officials have no knowledge of previous shock-related incidents related to the same components on the 7000-series cars.

Union representatives want Metro officials to immediately provide training for the people who work on the new trains, as well as perform emergency inspections on the trains to detect any possible defects.

The union said mechanics have never been trained on how to safely maintain the new cars, which feature significant design and mechanical differences from trains introduced to the system decades ago.

“As a result of this incident, ATU Local 689 is demanding of Metro not to bring any 7000-series trains into the shop until all employees that come in contact with them are properly informed on the potential for hazards, and training is given,” the union’s statement said. “Local 689 demands that 7000-series trains brought in for inspection be brought back into revenue service only after the necessary repairs are made.”