Fire Department says there were two trains impacted between two stations, Bowery and Essex Street.
Both trains were able to get to a station and the passengers got out at those stations.
A lower Manhattan-bound J train with 350 passengers and a Queens-bound M train with 200 riders had to back up to Essex Street to let people out. The MTA says that took about 18 minutes.
No injuries were reported.
The smoke condition started at 2:52 p.m. The MTA says a wall mounted electrical box between Bowery and Essex Stations caught fire.
The box feeds the third rail for the J and M trains. It caused smoke in tunnel.
Firefighters extinguished the blaze without issue.
After earlier suspensions and delays, service is operating normally.
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