You are buying a new HO 1:87 Promotex # 5523 Lowboy Tri-Axle Booster W/Integrated rear axle, 2 SETS W/Mudflaps. For all you Heavy Haul guys, this is a great addition. You must take a 5392, 5396 or 5513 Promotex trailer, cut off the awkward step on the back of the trailer and these will slide right in to expand your fleet. Comes with 2 sets - adequate parts for 2 trailers. NOTE: TRAILER IS NOT INCLUDED - YOU ARE BUYING THE TRIPLE AXLE BOOSTER SETS ONLY - TRAILER MUST BE PURCHASED SEPARATELY.