Your question helped me realise that I was making a rather simple and silly error. I went looking for details of the version I was opening and noticed that I was actually trying to open a very old version, 4.1.6, not the current version of RailModeller Pro 5.0.16 that I bought 12 months ago.
Suffice to say, the newer version opens just fine. Problem solved.
no worries - thanks for the clarification!
The latest version of RailModeller Classic will also work on OS X 10.4 - 10.11;
However the older 4.1.6 will not due to changes in newer versions of OS X.
The current release (4.1.11 as of today) can be found at http://www.railmodeller.com/older-apps.html.