Riders of the Blue and Orange lines, beware. Beginning Sept. 6, the train you board might not take you where you think. On that date, Metro will reshuffle the two routes in a way that even General Manager Richard Page can't quite get straight.

Hold on tight. All you have to remember is this:

Currently trains run from National Airport to Addison Road and back again. Starting next month the route will be National-New Carrollton. Trains leaving Ballston at present go to New Carrollton and turn around. The new route will be Ballston-Addison Road.

That means, for example, that a commuter accustomed to parking at Addison Road and riding direct to the Pentagon will now have to switch trains at one of the downtown stations, because the train he first boarded will be going to Ballston, not National Airport.

But the Pentagon worker who enters the rail system at New Carrollton, and is now changing downtown, will come out ahead: He will ride through to the office without a change.

There's another wrinkle: Trains on the run from National Airport to New Carrollton will be marked Orange Line. But coming back on the same route they will be Blue. The same procedure holds for the Ballston-Addison route. Trains from Ballston to Addison Road will be marked Blue; coming back they will be Orange.

Metro officials explain that having Blue Line trains running both ways on a single route would entail enormous expense because the entire system's color-coding would have to be redone -- the maps, pylons and destination signs on the trains. Moreover, as ridership patterns change, the system could revert to its present form.

So, riders will have to forget old ideas of where Blue and Orange lines run. Harold Barley, acting chief of rail transport, advises them to rely instead on the trains' destination signs. But he points out that most rides won't change -- only those that begin on an outer branch and end at another outer branch across town.

The purpose of it all, Barley said, is to pair two heavily used portions of track and two lightly used ones. Ridership surveys have shown that the current system is a mismatch: Orange Line ridership is heavy toward New Carrollton but light at the other end, Ballston. On the Blue Line it is heavy toward National Airport but light toward Addison Road.

Under the new matchup, extra cars needed for the National Airport crowds will also be full at the other end of the run, New Carrollton. Similarly, cars on the Addison Road-Ballston route can be reduced because demand is light at both ends.

But as ridership patterns change with the opening of new lines and more cars are delivered, the current system may be restored, Barley said.

Metro is now launching a media campaign to educate the public. The illuminated neighborhood maps in the stations will be replaced temporarily with posters explaining the shuffle. Kiosks will have 100,000 brochures. Loudspeaker announcements in the stations and trains will remind riders of the changes.

Even in Metro itself, many officials can't quite get it all straight. At a Metro board meeting on Thursday morning, General Manager Page drew gales of sympathetic laughter as he twice bungled attempts to describe how it would affect an imaginary passenger on the Addison Road line.

"Read the brochure, don't listen to the general manager, that's the right thing to do," Page finally advised. " . . . Passengers are going to have to pay a little more attention when they ride those two lines."