If you are asking about trains "running away" down the 4%, the C&TS has had none that I know of when I was there. There were a few incidents with work trains and such getting "a bit loose", but things got stopped with no damage done. Such is life on a mountain RR. Knowing when you are about to get in trouble and getting stopped or solving the problem before things get beyond your control is what being an engineer is all about.
We always had great respect for the Hill. You can't let it scare or intimidate you, but you have to respect it.
As far as the D&S goes, I know of a work train that got away coming out of Silverton in the early 1980's that resulting in serious flat spots on a K-28's drivers.
The Grande had few runaways on Cumbres. There is a pic in Ticket To Toltec of a derailment back in 1890's that looks like a pretty high speed pile up event. There are also pics in Dormans book of a Class 60 inverted and torn up pretty bad which would indicate it derailed at pretty high speed. If one looks at the derailment of the passenger train at Hurley's curve, it looks like there were moving pretty fast when they went into the ditch. But there is a difference between a "runaway" and simply "fast running"