Well, let me see. I am not going to go look up the density of western coal right now, but I can think of a 50 ton (payload) belly dump semi trailer that I saw a few of. It was on a tridem, however pulled by a 3 axle tractor for 6 axles total. If I think of how many yards it was rated I will post back.
Then I remember one particular triple set, grossed out at 300,000 lb. on 13 axles total. It was hauling some type of ore that I won't mention to protect the guilty. That was an off road haul, but the trailers themselves were used as singles at other times with 3 axle tractors to haul over the road loads (80K max).
Then there was another set of triple belly dumps that I remember that WAS over the road that grossed 180K on a special permit. That was coal too. When off the permit road, they dropped the third triple and operated as doubles grossing out at 120K.
Maybe these will give you some ideas of the relative capacities. The first was Al, the second and third ones mentioned were steel.
Tractors in each case weren't anything extra special-pretty much regular over the road rigs, but without sleepers.
rmw