ICE has a special restriction within 3000 ft horizontally and 1000 ft vertically of any ICE vehicle. [2]
Otherwise they're allowed to fly below 400 feet.
They're also loud as hell, and are relatively easy to notice because of their noise.
In most cases, they're treated exactly like drones, i.e. are limited to 400 feet. In a few exceptional cases, they can go up to 1200 feet.