In some lucky parts of New York, they build the subway first and wait for the neighborhood to catch up. That's what is happening on the far west side of Manhattan. The still-unfinished 7-train extension is already powering a massive build-out of commercial and residential towers.

But in other parts of the city, things work the opposite way. As older neighborhoods regenerate and flourish, local subway lines often struggle as waves of new riders appear. It then falls to residents, politicians and advocacy groups to hound the MTA for better service.

That's what is happening now along the Fulton Street spine of the A and C lines in Brooklyn -- around Crown Heights, Bedford-Stuyvesant and East New York.

Ridership is up significantly across the system. On each of 29 days last year, the number of rides provided exceeded 6 million. Yet service isn't up to speed.

This week, we learned that A and F weekday trains are late almost a third of the time, in part because of track work, according to MTA stats.

We also know trains are delayed because of overcrowding and unfortunate rudeness, such as riders who hold doors open and others who refuse to make way for others getting on and off.

C trains are notoriously crowded, dirty and infrequent, especially in the morning rush, the Riders Alliance, an advocacy group, has said. One reason: The A and C use rolling stock that's decades old.

Stations are also a mess. The Nostrand Avenue A and C station serves about 5.5 million riders a year and urgently needs more entrances and exits.

The MTA is trying to determine better ways to improve the system to "optimize the distribution of work" and minimize impact on riders.

And the 7-train extension? We need that, too. The MTA is building it with city money -- not its own cash -- and the project will help our economy. That's all good news.

Still, it's the MTA's duty to ensure that the subways keep up with the times and give beleaguered commuters the best ride possible. It has a way to go.