The winters in northern California can be pretty chilly, especially in the wee hours of the morning. The S.P. F units weren't too bad, but the GP9's leaked a lot of cold air around the cab door on the fireman's/brakeman's side. I recall that issue with the older Geeps but don't have any strong memories of being cold in the SD 40's and 50's and the ALCO six-motor units that we had on the San Joaquin Division.