a passenger train that is ridden primarily by passengers who travel regularly from one place to another
"The new commuter line opened today, offering residents a faster way to reach their daily workplaces in the city."
someone who travels regularly from home in a suburb to work in a city
"The crowded train was packed entirely with commuters rushing into downtown for their morning shifts."
One who commutes (etymology 1).
"The busy commuter rushed through the crowd, clutching his briefcase and trying to catch the last train home before rush hour ended."