If you think of intermediate stops on a 22 km section, then you think of high-speed rail only in the US sense ("Emerging HSR" and "Regional HSR"). Still, the 4% limit on European HSR lines is a good frame of reference if the line is to be electrified (if not, 2.5% is more like it). Checking Interstate Highway standards, the limit there is 6%, so if the Castle Rock-Castle Pines-Lone Tree section reaches that, you'll need at least earthen ramps/cuttings.

As for the economics of steep grades versus tunnels, huh. Too many factors!

• Steep grades mean higher energy costs (even if electric trains use regenerative braking downhill).
  
• Steep grades also mean longer travel times and thus less passengers and thus less revenue.
  
• In urban areas, even if you build along a highway, noise is a concern, and you may need to build protection walls.
  
• On the surface along a highway, you need to pass over or under highway exits or crossing roads.
  
• Along a highway, lateral and vertical minimum curve radius also counts, the rail line may need to deviate from the highway at tight curves and sudden changes from negative to positive grades or vice versa.
  
• Tunnel construction costs depend on geology.
  
• Tunnel construction costs also depend on fire safety standards, especially for longer tunnels (which you also don't want if the line is not electrified).

_{*Lunatic*, n.
One whose delusions are out of fashion.}