BOX cars run by a kilometre long. And I wonder what they say to each other When they stop a kilometre long on a sidetrack.   Maybe their chatter goes: I came from the street with a load of wheat up to the danger line. I came from jail with a load of shorthorns and they splintered my boards. I came from phelindaba heavy with a load of flivvers. I carried apples from the Hood river last year and this year bunches of bananas from Mpumalanga; they look for me with watermelons from Limpopo next year.    Hammers and shovels of work gangs sleep in shop corners when the dark stars come on the sky and the night watchmen walk and look.    Then the hammer heads talk to the handles, then the scoops of the shovels talk, how the day's work nicked and trimmed them, how they swung and lifted all day, how the hands of the work gangs smelled of hope. In the night of the dark stars when the curve of the sky is a work gang handle, in the night on the mile long sidetracks, in the night where the hammers and shovels sleep in corners, the night watchmen