"PLEASE DON'T EAT, drink, smoke or litter on Metro vehicles or in the stations," the sign says. But it doesn't say anything about dancing. So Saturday night at 6, if you happen by the Union Station Metro platform, don't be surprised if you find dancing on the platform, in the trains, on the escalators and anywhere else that strikes the fancy of the 10 to 12 creatively adventurous members of CatScratch Theatre. "Redline Revisited" is what they call this piece -- a combination of movement improvisation and guerrilla theater meant to take riders by surprise. The work will travel with the Red Line from Union Station to Friendship Heights, and anyone is welcome along for the ride.