I would use both HOG and TransDem, depending on whether I make a real world route or a fictional one (using tools like TerraGen, WorldMachine, Wilbur and so on - anything that produces greyscale maps). In the first case there is nothing but TransDem, and in the second case I would use Hog, at least until I find out how to convert my data to a format that TransDem can read.
For non-American maps I don't think HOG is very good, as there are no TIGER data available, but GoogleEarth can reach almost any point in the world.
There are still lots of railroads to be built in Trainz, mainly such that have been dismantled in the meantime, as for instance the lower part of the Andes route in Peru.
Dralex