The vehicles are locked from editing by default. In order to modify these to suit your own requirements you will need to right click on the vehicle name and select edit a copy.
One of the features common to large vehicles such as these is secondary steering. This is where the axles on the trailers have their own steering. This secondary steering can be linked to the main steering (limits can be set on the amount of secondary steering) or it can be independent and controlled by the driver as he is turning. It may also be a combination of both of these and in that case you will want the option to have rear steering but be able to independently override this at time of turning.
To do this you will need to make a copy of the vehicle as mentioned above, then right click on it and select 'edit using advanced editor'. Highlight the trailer unit and click on 'edit'
In the edit dialog box click on the 'Rear Axles' tab and then click on the 'Steering' button, then tick the box shown below and enter your max secondary steering angle and lock to lock time as shown below (these will be vehicle specific):
Now when you are 'driving' this vehicle in AutoTrack you can override the rear steering and independently control it as you are navigating your vehicle. To override you can click on the option below or use the wheel on your mouse.