At about 5:14 a.m. officers responded to the CTA Belmont Station in the Lakeview neighborhood, Chicago police said.
A man was found unresponsive in a train car. Chicago police said he was pronounced at the scene.
Chicago police are conducting a death investigation.
At first, the CTA said Kimball-bound trains were stopped on Monday morning due to a "sick" passenger.
By 9:30 a.m. it appeared trains were moving again.
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