Unskateable rail
The orange railing by the DIY-spot and mini-ramp under the big parking garage are ungrindable in certain areas. You just slide off or fall every time.
Ideas
Yellow rails on a loading dock in an alley in Gullcrest village are ungrindable, player treats them as if they are walls and automatically goes for the wall ride every time, on the off chance I do land directly on the rail it does not enter a grind.