We visited Hume doors, one of the door suppliers for our builder, early February this year. From that visit, we learned that there are 2 types of door tracks (above the sliding door) for sliding doors: the normal one and the heavy duty one.
The normal track looks like a "J" shape and the mechanism above the door only has one wheel. The heavy duty track looks like a "U" shape and it uses 2 wheels instead and is normal much stronger.
In the Hume doors showroom you can see all sorts of doors on display, and they even have one set of each sliding door, with all the covering removed for you to easily compare and understand the differences between the two. The photos below were taken from that trip.
First is the normal "J" shape track, viewed from the bottom, you can see that there's one piece of track.
From different angle, you can clearly see how the single wheel connects to the "J" shape track.
This is the heavy duty "U" shape track, from below, you can see there are 2 tracks installed.
This photo wasn't very clear, based on the "J" shape one above, just image there's another wheel on the other side, sliding on the other half of the "U" shape track, and that's roughly how it works.