If you are considering building a model train layout, watch this video.
Il discuss curves, grades and many other bits of vital layout knowledge and experience.
Step by step building a model train layout.
In North America, equipment for unlimited interchange between railway companies is built to accommodate for a 288-foot (87.8 m) radius, but normally a 410-foot (125.0 m) radius is used as a minimum, as some freight carriages (freight cars) are handled by special agreement between railways that cannot take the sharper curvature. For the handling of long freight trains, a minimum 574-foot (175.0 m) radius is preferred.[3]
Link to Wiki
https://en.wikipedia.org/wiki/Minimum_railway_curve_radius
To translate that into model railway terms, 288 feet becomes 3’ - 3 ¾” or 1.09 m in HO Scale
410 feet becomes 4’ – 8 ½” or 56.5 inches or 1.436 m in HO scale
574 feet becomes 6 feet 7 and 1/8 inches