These modifications are in free building instructions and an inventory/parts list is available for you to add this to your own 8258. By modifying 8258 with this you enable the ability to connect to soon to be released lowboy trailers!
This kingpin at the front of the trailer makes a pivoting connection with the tractor's fifth wheel. It forces the fifth wheel to open up and as soon as the kingpin is in place rubber belts forces the fifth wheel to close again. It stays there until it is manually released. By moving two levers manually the trailer can be released and thus uncoupled.
The pictures show two lowboy trailers. Both are so called detachable gooseneck trailers. The Yellow version has a liftable third axle. The Blue one a so called flip axle where a part of the frame holding the third axle is flipped on top of the second axle.
With this type of lowboy trailer the gooseneck is removed to make the deck accessible. Because the front part of the trailer rests on the surface the truck with the gooseneck still attached can drive off. In order to keep the gooseneck elevated at the correct hight is as a support. This support rests on the trucks frame.
Instructions for both shown lowboy trailer will be available soon, the first one in December 2014, the second one hopefully the 1st half of 2015.
Both lowboy trailers can perfectly be combined with LEGO® Technic set 8043 Motorized Excavator as well as with LEGO® Technic set 8275 Motorized Bulldozer. Obviously you can combine it with any of your MOCs as well.