It is all in the scale. There are many different scales that will run on our 45mm gauge track.
Standard gauge, 4' 8.5" between the rails, trains 1:29 ( incorrect scale, but made by many manufacturers) and 1:32 (correct, but not many manufacturers),
American Narrow gauge, 3' between the rails 1:20.3,
European Narrow gauge, 1 meter between the rails 1:22.5.
There are other scales out there that run on these tracks, but they aren't as common.
Chuck
It is all in what you like and want to run. I have trains in all the above scales with the exception of 1;32.