Trial Mountain just keeps giving once you really get the hang of it, I think it's popular because people have grown up with it, it's been in since GT1.
The turn mentioned above, keep on the left side of the track after the first tunnel then cut the corner as tight as you can without hitting the rock face, you shouldn't need to slow down too much for it, then nail the throttle on the way out, but not too early, you should be parallel with the rock face on the left, probably both left wheels on the curb when you leave the corner.
The next corner needs a pretty similar approach. The key to Trial Mountain is getting to realise how tightly you can cut a lot of the corners, but you need to know where to brake or ease off on the throttle for each approach too.