I worked as an executive for the MTA in the mid 1980s and am going to disagree with the "broken windows" theory, and say improved subway car technology and storage yard security made the difference. read more
They were the first new cars specifically designed to fight graffiti with the use of stainless steel inside and out, and washing off graffiti was MUCH easier on these and subsequent new subway cars. Second, subway yards for overnight storage were poorly secured so graffiti artists could work for hours spray painting. read more