Have you looked inside the ../Steam/steamapps/common/railworks/Install folder?
There is the complete dotNetFX40_Full_x86_x64.exe package that Steam will try to install on your behalf when first starting railworks on a new computer.
The other required 3-rd party stuff is there as well: DirectX, PhysX and Visual C redistributables.
Don't know about that Flash player. With Flash being a security risk it's perhaps better to keep Flash up to date through your prefered Internet browser.
Any of the tech-heads able to elaborate on this?