While the city’s lettered subway lines, with the exception of the L, are running old trains well past their expiration dates, the numbered routes have received many new train cars over the past few years. Now the lettered lines are receiving a high-tech fleet of cars and taking the lead in an ongoing rivalry between the two divisions for better service and more modern technology.

The split between the lettered and numbered subway lines predates the creation of the Metropolitan Transportation Authority. Two private companies, the Interborough Rapid Transit Company and the Brooklyn Rapid Transit Company, ran the two divisions as separate subways until 1940, when the city purchased both and merged them into one system. But the same trains cannot run on both line divisions because the company that owned the lettered lines dug tunnels wider than those on the numbered lines.

The varied history of the two divisions still shows itself today in the competition for capital from the MTA, which modernizes only one division at a time. After two decades of using trains that break down more often than those on the numbered lines, the lettered lines may finally be gaining the technological edge.

The cars making their debut on the N and Q lines, which are similar to those running on the no. 6 line, feature high-tech video displays that track a train’s location along its route. Straphangers say the ride feels smoother than on the older cars and that the automated voice announcements sound clearer than those on the no. 6 trains.

The entire fleet of 660 new R160 cars, valued at $952 million, is scheduled to be fully operational on the lettered lines by May 2008.

The numbered lines have historically been the first to receive technical upgrades from the MTA. While the numbered lines received new cars about five years ago, the C and the E lines are still equipped with trains that date back to 1964, a Transit Authority spokesman, Charles Seaton, said. A subway car is built to last about 35 years, but Mr. Seaton said the MTA’s maintenance procedures have extended their natural shelf lives.

“There have been lines that have been favored and lines that have gotten screwed and ignored,” a coauthor of “The Subway and the City: Celebrating a Century,” Stan Fischler, said. “A lot of it is political.”

With the exception of the L line, which the MTA uses as a testing ground for new technology, lettered lines have not received a single new train model since the 1980s.

“When the modern era of capital repairs began, they started with the no. 2 and no. 3 lines,” the staff attorney for the Straphangers Campaign, Gene Russianoff, said. “Back in the 1980s, the trains were covered in graffiti and looked like the equivalent of abandoned housing. The new trains on those lines were like the cavalry to the rescue.”

The no. 7 line was, for a long time, always first to receive technological upgrades from the MTA, another co-author of “The Subway and the City,” John Henderson, said. “The no. 7 ran through high-quality neighborhoods, so a lot of new stuff arrived there first.”

Although the L has since replaced the no. 7 as a technology leader, the numbered lines will not be behind the technology curve for long. By the end of the year, stations along the numbered routes will receive the train-arrival boards recently rolled out at 14 stations on the L line. The MTA has no plans to bring the arrival information screens to any of the lettered lines.

The MTA currently runs 6,200 subway cars over 66 miles of track. The Second Avenue subway line, construction of which is expected to conclude in 2013, will join the historically neglected ranks of the lettered lines as the T train. Because it will share a tunnel with the N and R lines at 72nd Street, the T will run the longer, wider trains on its two tracks.