The actual gauge of the track and rolling stock is immaterial. The attachment points (a.bog0 etc.) run on an invisible track line and what you see is "window dressing". You can mix and match how you like without any regard to reality. For instance in real life 2ft6in, 750mm and 760mm gauges are sufficiently different to prevent interchange of stock. In Trainz you would be hard put to see the difference. Similarly I sometimes mix metre and 3ft gauges as the difference is only really visible close up. If you really want to, you could run 15in gauge stock on Brunel's 7ft track or vice versa