Because so much of the Rio Grande is well over 5000', the horsepower losses of a roots blown diesel are unacceptable in most instances. The turbocharged GP40's are able to overcome the less dense atmosphere in throttle notches over 6 where the turbo disengages from the flywheel and is driven purely by exhaust gasses and is able to force more air into the cylinders than a roots blower fixed to engine speed can.