The Metropolitan Transit Authority says the incident occurred at about noon on Thursday. There were no injuries reported.
The MTA says the train's crew lead about 750 passengers in single file to the first car then onto the platform after the fire caught underneath the sixth car of the train.
Firefighters put out the fire. Full service has since been restored.
The MTA says its police are investigating how the mattress got on the tracks and believe it may have been placed there as an act of vandalism.