THIS COULD HAVE BEEN BAD! Transmission overheat light came on during downhill, a steep long grade, on west side of Eisenhower tunnel (I-70 Westbound) and transmission failed to engage and vehicle started going very fast as was now a runaway. To keep brakes from catching fire... coasted as a runaway vehicle rest of the way down and then up I-70 until vehicle stopped (according to CDOT/highway warning signs of what to do in case of runaway vehicle). Some smoke came from vehicle, no obvious fire, and had to engage parking brake to keep from rolling back down the hill. State patrol and tow notified, local tow driver came and helped me by driving to local Autozone and bought and had to use three (3) quarts of fluid. Tow driver followed us to hotel a few miles away and I am now trying to figure out service issues 100+miles from home. This vehicle had been checked a few weeks prior by a service center and fluid level and color were fine!