The subway system is an inherently dirty place, but train cars are getting filthier, according to a survey released Thursday.

Straphangers Campaign, a transit riders advocacy group, said 42 percent of the train cars surveyed in 2013 were clean, a 10-point decline from a similar 2011 survey. The worst offender was the D train, a line where just 17 percent of cars were clean. The cleanest ride in the system goes to the L train, where 63 percent of cars surveyed were dirt-free.

"The MTA's maintained the same level of cleaners although ridership's gone up," said Jason Chin-Fatt, a field organizer with the Straphangers Campaign.

The Straphangers Campaign looked at 100 subway cars on each of the 20 train lines between September and December at rush hour, evening and overnight hours during the week and on weekends. The level of cleanliness ranged from scuff marks and ground-in gum spots to dry sticky spots and food on the floor.

"We focused on the grime," Chin-Fatt said. "It's something that riders can readily see or smell from the ground or the seat."

Of the 20 lines surveyed, nine saw a significant decrease in train car cleanliness, while the rest were largely the same from the 2011 survey.

New York City Transit disputed the Straphangers Campaign's methodology, arguing that the agency's own surveys are more statistically accurate. NYCT surveys a minimum of 250 cars per line over six months and includes the amount of litter strewn about a car and graffiti, the MTA said.

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According to the latest MTA statistics on subway car appearance, cleanliness slightly improved systemwide last year compared with 2012, with the biggest improvements on the C, B and J and Z lines.