It requires the Trade advance. Standard cost is 60 resources. Strength is fairly insignificant at 0/6 and speed is the standard minimum 1.5.
Unlike the Civ2 freight unit, its incorporation in something else being built is not restricted to Wonders. It can be used to speed the production of anything except colony ship parts. However, like units and buildings being incorporated into new ones, it contributes only two-thirds of its resource cost.
Cheaper than cash for a rush job, but should still be used sparingly. If you change your mind part-way through building a freight unit, you can't get all your resources converted to gold by simply switching to a building or Wonder: you must sacrifice the one-third of its resource cost, effectively redeeming the completed part of it there and then.