One other note . . . if you're on a snowmobile trail be sure to pay attention to approaching snowmobiles. While most two-stroke snowmobiles should be loud enough for you to hear them approaching, four-stroke snowmobiles are notably quieter and can "sneak" up on you if you're not paying attention. If you intend to hike, ski, snowshoe in on a snowmobile trail, listening to your MP3 player at the same time is probably an unwise decision.
Finally, remember that the snowmobile trail was built, signed, brushed and groomed by volunteers associated with snowmobile clubs for the purpose of snowmobiling and as such snowmobilers have every right to be on the that trail (assuming it is a legal trail). If you see a snowmobile approaching you good etiquette dictates that you get off the trail and allow the snowmobile to pass . . . waving (with all five fingers) is always appreciated and if you have a dog keeping the dog under control is always advisable. If you follow this advice you should find most snowmobilers to be friendly and a helpful lot.
On a side note . . . where I snowmobile I have run across folks hiking, walking their dog, snowshoeing, skiing, riding their mountain bike and running their dogsleds down the trail . . . I am pleased to say that I have never had any problems with any of these folks as I am a firm believer in multi-use trails and believe that some trails not designated as multi-use trails such as snowmobile trails and ATV trails should be used by others who also enjoy the outdoors -- whether they choose to engage in motorized or non-motorized activities. In the past I have often stopped to give directions or to help out (most recently helping a dogsledder who had a pack of dogs that were trying to go after a porcupine walking across the trail.)
In all cases I have never had a bad experience . . . unfortunately that isn't always the case. In areas of southern Maine I have heard stories of folks flipping sledders the bird, deliberately staying on the trail, etc. even though these trails are, as mentioned earlier, designated expressly for snowmobilers. That said . . . I have also sadly heard stories about sledders roosting folks with snow, flying by walkers at unprudent speeds, etc. Bottom line . . . I believe that if all folks respected other folks on the trail it's good for everyone.
"Courage is not the absence of fear, but the realization that there is something more important than fear."
"Death is only one of many ways to die."