There was a crusade against snowmobiles, started around 2000, against 2 stroke engines being in the park.
Most, if not all, of the snowmobiles used in the park are now 4 stroke (same as automobiles) BUT it's also required that one must hire a guide EVEN THOUGH THE ONLY AUTHORIZED TRAILS ARE THE SAME ROADS THAT AUTOS, RV'S and MOTORCYCLES use in the summer. This is not fair and is unreasonable. Having been there in 2000, riding a two stroke machine, the animals were not afraid, we stayed on the road/trails and there was no smoke (except at the gate at West Yellowstone and at the gas station). This is a park that should be open to all. The snowmobilers are not doing any harm.