The easiest combination is SRTM for DEM terrain and Open Street Map (or another Map Tile service) for maps to find your railway. So, if you follow the Quick Start Guide in the TransDEM Trainz manual, it should take you there, even if the Map Tile tutorial in the manual is for a place in Germany, half around the globe from Australia. There are more advanced geo data sources which may be specific to a region, however, I recommend against them until you have accomplished results with the simpler data sources.
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