Marty Bernard's journey to Quebec City in August 1970 gives us a good idea of locomotives working there on a typical day. He bothered to photograph almost every locomotives he encountered that day, making it an almost perfect snapshot.
Using his pictures, I was able to get a quite precise idea of the roster. That way I was able to figure out how many locomotives of each type could be found at Quebec City. Then, I scaled down the number of units to represent the fact our layout is about half the size of real life operations.
Number per type reads as 2 (1) .
2 stands for the number pictured by Marty Bernard.
(1) is half that number to fit the layout
 is the adjustment I make to make it more realistic
Then some pertinent remarks as needed.
C424: 3 (1.5)  - I rounded at 2 because they always run in pair and 1.5 is practical! I have one Atlas in wet noodle [#3214]
and 1 Atlas-Kato in zebra stripes [#3201]
(may need a repaint or to get another one in the right scheme since Atlas Classic locomotives are better detailed).
GP9: 6 (3)  - The work horse at Quebec City. I have 2 new Bachmann waiting their new paint scheme [#4492 & 4521]
and probably will need another one, probably a Bachmann to have a consistent fleet at low price. Or maybe a Proto 2000 in green/gold livery since there's photographic proof one still operated at Limoilou in 1967 [#4509]
RS18: 3 (1.5)  - I'll have to get and bash an Atlas Classic RS11 [#3714]
. I already gathered the detailing parts. Since it's a serious and costly bash, I don't expect to make a second one.
S4: 2 (1)  - The club has an Atlas GT S4 that will be detailed and painted to fit one of Quebec City switcher [#8176]
FPA4: 4 (2)  - The main passenger locomotive in Quebec City. However, since we don't model Palace Station, I feel it's useless to get more. I already have a Proto 2000 PA4 in CN livery that will make a good stand-in [#TBD]
B-units: 2 (1)  - EMD and MLW, only needs one to complete a set [#TBD]
. Since FPA4 often pushed short local trains in the 70s, I'm not in a hurry to get one. Maybe a Rapido in the future, who knows.
Steam Generator: 4 (2)  - Lots of freight locomotives pulled trains in Quebec City area, mainly on Murray Bay Sub [#TBD]
. Since we don't operate passenger trains, getting one from Rapido would be more than enough.
Budd: 1 (0.5)  - Acquiring a Budd in the future would be a nice move to get the feeling of the era. But it's not a priority so far [#6116]
Wood caboose: 2 (1)  - I already have a True Line caboose in wet noodle. Since we operate many freight trains, I have also built a Sylvan Scale resin Pointe Saint-Charles caboose that also fits the era.
RS10: 2 (1)  - We already own a Proto 1000 in Multimark scheme.
RS18: 2 (1)  - Building a RS18 is a costly and lenghty endeavour. If I can grab a Proto 1000 specimen in Maroon & Grey scheme, it would be a possible way to add it to the fleet.
S2-S3: 2 (1)  - I have a Proto 2000 S3 in Maroon & Grey scheme, block lettering).
Budd: 2 (1)  - Would be a nice future addition to depict passenger operation at Saint-Sacrement Station.
Wood caboose: 1 (0.5)  - I own a True Line version in Script scheme as found at Palace Station on 1970s pictures.
Steel caboose: 1 (0.5)  - We've got a Rapido Transcona caboose in Multimark scheme as seen on era pictures.
National Harbour Board:
GE 44-Ton: 1 (0.5)  - I already repainted a Bachmann Spectrum in the correct paint scheme.
Other locomotives will be added to this preliminary roster. In fact, other locomotives such as M630C weren't unusual on CP Rail intermodal trains to Wolfe's Cove. I already got one from Bowser. Also, M630C were also a common sight in Quebec area, pulling long intercity freight trains, including grain trains. I also have two Bowser unit in wet noodle and zebra stripes schemes.
Finally, GP38-2W, GP40W and M420 were often seen in the mid-1970s. So far, we got two Atlas GP38-2W in zebra stripes.