The crane has bogies at both ends of a horizontal track. At least one wheel in each bogie is mechanically coupled to a remote-controlled motor, causing the gantry or portal to travel along the track. There is a detector on the gantry operated if it runs out of true at either end. A control signal proportional to the amount of deviation is sent to one or both motors, producing a correction to reduce this deviation. There can a component monitoring the position of each end of the gantry on the track, and generating a signal dependant on this position. These two signals are combined to form a signal dependant on the deviation at both ends.