A cyclocross bike has a slightly higher bottom bracket than a road bike, and much better tire clearance. Most currently fashionable road bikes won't fit the wider tires necessary for trail riding. I don't think that road bikes are particularly fragile, but my cyclocross bike seems to be built a bit heavier than a comparable road bike frame.
I use 42mm wide tires when on trails on my Specialized TriCross Comp, and 23 mm tires when riding just on the road. But my bike is aluminum, not steel, so it probably has a slightly stiffer ride than the Surly.
Another similar bike is the Red Line Conquest. Perhaps you should ask your question on the cyclocross forum.