An Improvement over RRT 2
There's a new economic model and a 3D layout which is nice (no more straight and 45 degree pieces, it's point to point lay out)
The challenges are pretty much the same old thing: 50 Million in personal worth, be only company after 30 years, 30 million in company worth, haul 120 loads of oil, etc.
Bridges and tunnels are a big improvement, but it can be hard to snake around mountain sides like in RRT 2.
All in all a nice evolution, nothing revolutionary (economic model is quite different , as I have already said).
Sid Meier's Railroads has been out for a while for the PC I haven't seen anything on a Mac Port, but then I've been too busy with WOW to notice.