From the mid-1970s, when Metro was born, through 2008, the agency bought six batches of rail cars, a total of 1,134 cars, nearly all of which are still in service. The oldest, the 1000-series, dates to the opening of the subway, and newest, 6000-series, arrived a decade ago. Notice how all the cars more or less look the same, regardless of age? That’s because Metro wanted aesthetic consistency. It also wanted each new batch to be mechanically similar to the batches that came before, so that cars of different ages could be coupled together in trains. But maintaining mechanical compatibility meant limiting technological upgrades as each new batch came off the assembly line.

PostTV catches a short peek at new Metrorail cars including doors that won't trap riders who move a little too slow. (Lee Powell/The Washington Post)

Now, finally, Metro is about to begin phasing out the old cars in favor of a new generation, the 7000-series. These cars are so advanced, they will run together as separate trains, never coupling with the current fleet. Sixty-four of the new 7K cars are due to be in service by summer 2015, and 748 by 2018. One set of four 7K cars has been undergoing tests on Metro tracks since the start of the year. Tuesday morning, we went along for a ride.