The most useful tip is: do tutorials.
There are hundreds of beginner tutorials for Blender, which cover all you will ever need to know for modelling scenery for Railworks.
Blender has hundreds of functions but you will actually need just a handful of them. So you want to try tutotials covering the GUI, the basic tools and commands, how to manipulate objects, how to make a cube, etc.
Then you can add the Bigex exporter (or the other exporter in UKTrainSim forums), and do some railworks-specific tutorials such as
this one. I should be able to find the other Railworks-specific tutorials I used, but Google will help you.
The modeling part is actually easy and addictive in Blender. It has a reputation for a steep learning curve, but I did not experience that, and of course it is not a problem if it is your first 3D modeling program. The learning curve, for me, started at texturing and UV mapping. But it was not Blender's fault, I just had a hard time figuring out the concepts of material, texture, mapping, unwrapping, etc. After doing it once using tutorials, it became quite simple.