This question about the new Metrorail cars came in during my online chat today, but I didn’t get a chance to respond till now.

Metro 7000 series cars: “So I’ve read the press releases and news articles regarding these new cars and it seems like they’re putting in some good designs (poles on every seat, non-skid flooring, better signage, etc). I noticed that they say that the cars are matched in pairs of four rather than two. So wouldn’t this mean that they would lose the option to run six car trains and be forced to run eight-car or four-car trains instead? If so — here’s hoping they run eight-car trains!”

Those new train sets won’t be able to operate in six-car configurations. This is one of the most visible changes in the 7000 Series. The entire design represents a sharp break from the past. (One thing you won’t see, because of the design and technology differences, is a 7000 series set of four married up with an older set of twos to make a six-car train.)

Dana Hedgpeth, the Post reporter who covers Metro transit, wrote last week about Metro’s design effort with the new cars, including the poles, flooring and signs. We’ll have more about what to expect from the new cars on this Sunday’s Commuter page in The Post. Let me know in comments below if you have more questions about them.

The cars still are being designed. Metro won’t receive the prototype trains to test until the summer of 2013. The new trains should enter regular service in time for the opening of the Metrorail extension through Tysons Corner to Wiehle Avenue at the end of that year.

That doesn’t mean you’ll have to ride the new line to see the new cars. Any type of Metrorail car can operate on the new line, and the 7000 series can operate anywhere in the Metro system. Metro hopes to buy 428 of the 7000 series cars. Eventually, the new cars will replace the 1000 series, the 300 original cars in the Metro fleet.

There’s no guarantee that we’ll never see another four-car train, an image that makes riders shudder. But that’s not the direction Metro is going in. Rather, the goal is to get to an all-eight-car fleet. At that point, the system would be about maxed out on rush-hour capacity. We can’t make the trains any longer unless we make the platforms longer. In most cases, Metro is operating all the trains at rush hour that it can.

Metro did slightly widen the gap between Red Line trains last year to increase the efficiency of the line. (At the same time, the transit authority added cars to create more eight-car trains on the line at rush hour, a move that I think has worked out well, and which there’s no point reversing.)

At rush hour, Orange and Blue Line trains compete for space in the Rosslyn tunnel, so the transit authority plans to divert some Blue Line trains to the Yellow Line bridge, the only other way for trains to get across the Potomac.

The most likely issue during future rush hours isn’t the possibility of returning to four-car trains, but rather figuring out what to do when we’ve filled up all the eight-car trains.