Possible Clues:

• Box made of cardboard
• This might hold the weight of the vehicle
• One of those from the boxing industry in ring missing from comic strip
• Paintings found in prisoner's box
• Sydney might have been a novel sort of boxer that lost his head
• Vessel to load?
• Might Sydney box with the weight of the vehicle?
• Barrister who lost his head in Paris
• That's what your bus weighs, just in case
• By the sound of it, wait for the box car