From the article:

The researchers found that the rubbing of metal wheels and brakes on the rails releases metal particles, which appear in particular concentrations as the trains arrive at a station.

“We noticed that when a train arrives at the station, the concentration in the air shoots up, and about 15-20 seconds after it leaves, the concentration slowly comes down,” said Ghandehari. “This implies that it is the stuff that’s at the bottom of the tunnel that gets churned up and deteriorates the air.”