Chris:
I thought it might be mid-late 50's as CN ran a museum train at that time, but I'm not sure about the diesel paint job -- I think that came in the 60's.
It might be in conjunction with the opening of the NMST in Ottawa -- 4 of the locos went there. I think 6167 went to Guelph in 1967. I think NMST opened in 1967, so that sets an end date.
Except for the diesel, all of the locos exist today. The current 5700 started as 5703 and was renumbered when someone scrapped 5700 in error. Museum wanted 00 numbers (they weren't planning on last 2 digit DCC )
What a beautiful sight! Surfin around the net I found information that looks credible with regards to when this pic was taken: Railway Week June 22-28, 1964 in Belleville, Ontario.