This was reported by another user in the Xbox forums so I thought I’d try it myself. Checked my stats before doing any services and it stood at 4,142.16. I then drove a service on GWR, with the 166 brand new out of the box with zero mileage on it. After the service, the train (on the stats page) shows 35.87 miles. I kept within the speed limit the entire time, so this should have increased my total mileage to 4,178.03. However, my total mileage is now 4,180.12 (2.09 miles better than it ought to be). I have completed the tutorials on GWR, but none of that mileage (which is negligible) was recorded against any of the trains or the total mileage. This might need investigation as the mileage should be accurately recorded by the game. Otherwise, how many miles do I know I’ve actually driven
Really? You'll never know, because this issue is as old as the whole game. The preserved content has been converted to be playable with TSW2, but that doesn't mean that old issues would have been fixed. And I wouldn't expect that it gets fixed at all.