They are just lines on a graph, but for many subway riders they will provide unique insight into one of the great aggravations of life underground: why trains on some lines are so often both crowded and late, while on other lines the trains seem to cruise along on schedule with almost no one on board.

In an unusually candid effort at self-examination for a habitually insular agency, New York City Transit yesterday presented what could be called an index of straphanger frustration. It made an analysis of each subway line that shows at a glance how often trains run late, how crowded they are and whether more trains could be added to ease the problems.

What is revealed is both predictable and eye-opening. Many subway lines are simply maxed out, meaning there is no room on the tracks they use to add trains that could carry the swelling numbers of riders. And that has implications that range from day-to-day decisions about how trains travel through the system to long-term planning on how to best move people around a growing city.

“From my point of view, this is scary,” said Howard H. Roberts Jr., the president of New York City Transit, who presented the data to members of the Metropolitan Transportation Authority’s board. “This is scary in the sense that right now, on a lot of these lines, we’re several years and a big capital construction project away from being able to provide what I consider adequate service. We’re constrained.”