Subway ridership in New York is in the midst of a resurgence almost unimaginable in the 1970s and ’80s, when the system was defined by graffiti and crime. Ridership has steadily risen to nearly six million daily riders today from about four million in the 1990s.

But the subway infrastructure has not kept pace, and that has left the system with a litany of needs, many of them essential to maintaining current service or accommodating the increased ridership. The authority’s board recently approved $14.2 billion for the subway as part of a $29.5 billion, five-year capital spending plan.

On the busiest lines, like the 7, L and Q, officials say the agency is already running as many trains as it can during the morning rush. Crowds are appearing on nights and weekends, too, and the authority is adding more trains at those times.

The long-awaited opening of the Second Avenue subway on the Upper East Side this year will ease congestion on the Lexington Avenue line. Installing a modern signal system, which would allow more trains to run, is many years away for most lines.

In the meantime, the agency is doing its best to keep trains moving on the century-old system. Workers known as platform controllers have been deployed at busy stations like 86th Street to direct crowds so that trains can depart more quickly.

Subway guards, the early-20th-century forerunner of today’s platform controllers, were posted at busy stations the last time the system had this many riders, during the Great Depression and World War II era. That role, The New York Times noted in 1930, required the skills of “a football player, a head usher, a stage director, pugilist, circus barker and a sardine packer.”