The raising of the outer rail gives a train a twisting effect as it transitions into and out of curves, and this twisting can also be achieved on a model, by lifting the outer rail too. The trains look more realistic as they go through curves simply because they behave more like the real thing.
The European association of Modelrailroader and Railroad Friends have a standard (NEM 114) for the amount of superelevation that should be added at various scales and they suggest 1.5mm for HO scale. Personally, I find this too small and I prefer 2mm to see a more noticeable effect.
To give my tracks the tilt I wanted, I made a bunch of tiny wooden wedges that I could insert underneath the roadbed of the track.
In order to make such thin wedges, I placed the blade of my table saw slightly off vertical. I then pushed some pine lumber (2x4) through the blade. I then flipped the lumber over and made a second cut, producing a fine wedge. Every flip and cut produced another wedge.
I inserted the wedges along the outer edge of the curves, and glued them down once I was happy with the track. Here one can see the wooden wedge sticking out along the outer edge of the track. (Each track gets its own wedge, do not put more than one track on the same slope.)
I then landscaped over them with drywall plaster, and ballast, etc.
Here you can get an idea of the angle with a steel rule lying on both rails of the inner track.
The tilt of this train is noticeable.
Notice how the outer rail of the inner track is higher than the inner rail of the outer track!