Abstract: A system for positioning a plurality of carriages to preselected positions across a supported article includes moving a train of detachably interconnected carriages with an engine carriage functionally connected to a driving facility, and simultaneously detaching selected ones of the carriages from the train at predetermined positions across the article, in a sequential manner, beginning with the selected carriage farthestmost from the engine carriage, and holding the selected carriages to a member bridging the article. The positioning cycle is completed by stopping the engine train at its preselected position. Thereafter, a work operation is performed on the article by moving a work operation performing means associated with each of the selected carriages from an inoperative or retracted position to an extended or operative position.