Miningman
Thanks for all that Jones1945.
The streamlined Northerns highballed past our station in Burlington, Ontario frequently. While many trains stopped these felllas never did. Folks milling about on the platforms paralleling the inside tracks waiting for their train would be in for a real treat or freightening shock when the 6400's roared by all raw power, always on the outside tracks, a very impressive sight.
My first encounter with them was this way and I was held captive by the moment, its such a vivid memory.
You know, I can't help but think that the railroads really knew what they doing back then. It was important and permanent. It was for all of us, it was service that could be counted on, all the time. It all worked so well, with great good people. We lost too much too quickly. It's just not like that any more.
The streamlined Northerns would take your breath away and capture your imagination. A person was frozen in the moment.
Don't mention it Miningman! I understand that the feeling when a railfan encounter his favorite big steam running in front of him, it can be almost as powerful and breathtaking as meeting ones first love! I believe you remember what Raymond Lowey shared about his story when he saw his works running at speed in person first time. The vibration , the heat and the energy, the sound and "sweet-smelling", passengers’ excited reactions made it an unforgeable moment to many railfan like you and me. Until nowadays, I still can't find any other type of railroad motive power have the same level of charms as steam locomotives!
CNR Class U-4-a is one of my favorites 4-8-4s, I love them since I found pics of them in books and stamps when I was a child. The design of U-4-a included wind tunnel testing to looking for ways to improve smoke clearance around the locomotive cabs of steam trains, instead of making changes to their existing "Confederations" Class’s designs and it proofed that CNR's semi-streamlining approach on their Class U-4-a were working fine. (Did PRR take notes?)
I suspect that CNR Class U-4-a inspired PRR about the design of Q1 #6130 Duplex’s since earlier 4-8-4 of CNR like U-2-d and U-2-h with smaller diameter drivers or even U-4-a itself were seen hauling freight trains, coincidently Q1 and U-4-a were both semi-streamlined, their diameter of drivers were also 77 inches. Please note that streamlining a freight engine was never a tradition of PRR, before or after Q1, although people could argue that Q2 were also semi-streamlined with their "T1-style boiler casting”.
M636C
But 6401 had no Royal Insignia, and was still lettered CN so is most likely to be green and black. What is hard to tell is exactly which areas were green and which black. I think CPR had a standard Hudson on its pilot train, to take over if the the blue one failed (which it didn't).
I am amazed at the collection of illustrations Jones 1945 has found, but none of them indicate 6400 in Royal Train colours in a real colour photo.
lol, I thought I found the pic you wants, Peter! This case reminds me of people were discussing about PRR's DGLE in different era on different trains or the color of PRR#3768 when it was pained in "bronze" shortly which had no photo avalible for reference. 
