Actually, the letter designation in A, B or C trains specifies the type of connection between the two trailers. A trains typically have an 'A' shaped dolly that connects with a pintle hook. B trains have a lead trailer with an extra two axles under a 5th wheel. C trains are really rare as they are costly upkeep, but essentially its an extension off the lead trailer to a 5th wheel, causing lots of wear on tires during turns.