Here is a great shot of the inside of the Colonie Shops dated February 01, 1981. It looks like three Alco locomotives are being worked on. The RS3 looks to be #4199, and one of the two C424s is #452. I can't read the other engine number.
I remember talking to friends who visited the D&H shops in the 1970s and 1980s and they said that as long as they signed a waiver they could tour the D&H facilities.
However, there were several rules that all visitors had to agree to, including: (1) no flash photography inside the building (which could distract the employees); (2) no climbing on the equipment; and (3) no jumping over the track pits. There may have been others, but those are the ones my friends remembered.
Those days are long over.