Abstract: An expansion linkage effects nearly perfectly straight translation of an ansion module relative to a trailer. The linkage has two relatively longer cross arms pivoted together at their intersection. One cross arm is additionally pivoted at its mount to the trailer and the other cross arm is additionally pivoted at its mount to the module. At opposite ends from the mounts, the cross arms have relatively shorter linkage arms pivoted thereto. The linkage arm pivoted to the trailer-mounted cross arm also pivotally connects to the module, and the linkage arm pivoted to the module-mounted cross arm also pivotally connects to the trailer. The intersection of the cross arms is 2/3 to 3/4 the distance from pivot axes at the cross arms' mount ends to pivot axes at the cross arms' opposite ends.