A heavy hauler is a very large transporter for moving oversize loads too large for road travel without an escort and special permit. A heavy hauler typically consists of a Ballast Tractor and . Some trailers may have independently steerable wheels, and several might be towed by one or more tractor units in a train. Self-propelled modular transporters (SPMT), some featuring a dozen or more self-steering axles with scores of rubber tires to spread out a load, are increasingly being manufactured. Working in coordinated teams, heavy haulers are able to carry loads exceeding 100 tons.