Hahaha, that is right, you press Triangle to grind on the Tony Hawk games. Well, the diference is that, cactus scrimblo slams down when you press square while in the skate right? And we need to slam down into the rails. In tony, when you press triangle near the rail, the character will "magnet" towards it. Then you just controll the balance.
Curently I had trouble getting the grind working with cactus. Since you need to slam down, you need to be over a certain height, or it will just miss the grind. Also, even with the right height, the grind would just not happen some times. Like it would slide off it.
I think going with the winning formula, of, press the button, magnet towards the rail, is the best approach. You could keep the slam down, but whenever on the air, and nearby a rail (check with an 3D area/volume), if you press the button, you grind instead of slamming.
I think that's how I would do it. Hope you find a solution!