I have designed a few layouts in my time and used the Hornby plastic quarter scale pieces (never enough pieces for what I'm doing) and their stencil from the 1980s and also no aid at all, just sketching out a rough plan and placing the actual track and experimenting with it. The trouble is, the latter takes ages to do, unless you're designing a very simple layout.
This Trackmaster software allows you to pick items of Hornby track from a menu and just move it around on the screen and rotate it to the angles that match the Hornby track geometry. The pieces snap together quite easily (not sure what "Muds" was on about saying that the software doesn't allow him to snap one piece to another if two track ends are close together) all you do is drag the track piece to the far side of the one you want to snap to and the little green elastic line shows you where it is going to snap. It's very easy. I think this would also be great for children to learn about how railway track works, although it is not a game or toy.
Anyway, whilst maybe not as powerful as other track design software out there (I have used Xtracad) it certainly is the easiest to use. The former is a nightmare just to get into. We have been waiting for Hornby to bring us a 21st century way to design reasonably complex layouts, utilising all of their set track pieces and platforms.
The results also just look nice, having used a photorealistic track pieces.