an engines redline

This answer varies a bit by authors.

Red Line is max. engine operation. For saftey purposes, the factory will set it in a safe zone where the car/engine can reach this "Peak" safely time and time again. And this is a good idea. Not only for you, but the factory as well as to keep cost and warranty work down.

On a race engine, it's not peak torque, but peak HP. Theres also a variant to this thinking as well. Without getting into detail, the max RPM engine will see is deterimed by the cams profile and abilty of the springs to control the valves actions determined by the cam.

If the springs can not keep the valves from doing there job, the power will fall off in a big way.

In general, a "Red Line" of any engine should be where peak HP is made.