A quick note on SCARM and AtlasO 3R track. I have been using SCARM for a week and for the most part I like it. Once you learn how to use the software, it becomes easier to use.
There are three areas I am concerned with. First, SCARM uses the long O-54 turnouts. Something to consider if you have the short version and do not use the included 4.5" piece included with the turnout.
Next, AtlasO bridges are not included in the available track library. To draw a bridge, you highlight the section of track and select the bridge feature. In the 3D view, the bridge appears as a flat section. In order to make a bridge less than 40", you must draw a custom piece of track, otherwise the 3D viewer will draw a bridge support irregardless of what is below the bridge.
Finally, drawing a custom piece of track is a PITA. Custom pieces are created by selecting a piece of flex track. That limits the length of the track to 40". If you are attempting to connect two pieces of track with a custom section, and the tracks are off by a hair, the custom piece will not be drawn.