My school got to go "behind the scenes" of the Hulk as part of a physics/calculus field trip. I'd seen this thread before I went, so I made sure to ask what was used to launch the coaster.
The ride mechanic who led the tour said the tires used to launch the train are air-filled, not solid rubber. He said they have to check the air pressure of the tires daily, and that on cold days they often have to add more air due to the lower temperature.