We see mighty, grandiose post offices prolific in every city from the 19th Century. Just why they needed to be so massive with elaborate ornamentation escapes me, for a utilitarian purpose. Newer built post offices serve the purpose of housing a counter and sorting and holding warehouse out the back, in basic structures.

I'm not removing the wonder of the older architecture, more why those magnificent buildings were designated as Post Offices?

Horse harnessed to that carriage is definitely thin according to the Equine Body Condition Score Chart. Too much rib showing; supplemental feeding for this working animal seems in order.

Interestingly, the horse drawn carriages, or 'caleches' as the locals call them, were recently banned in Montreal (on January 1 2020) following a series of unfortunate incidents.

L-R vis-a-vis and express wagon

The current name of St. James Street, in French. The six-columned building on the right with the wonderful pediment is indeed the Bank of Montreal, still open, with a bank museum inside. If you could swivel to the left ninety degrees, you would see the view featured in this Shorpy post from 2018: Place d’Armes: 1916.

Isn't this just up the block from the Infirmary?

The splendid Second Empire Style building to the left of the Bank of Montreal is the Montreal Post Office, built in 1872-1876 by the Canadian Department of Public Works and designed by the supervising architect, Henri-Maurice Perrault. The Parks Canada website describes it as "one of the finest examples of Second Empire to be found in Canada," and goes on to state that "the federal government, intent on making its presence felt, chose, for the Post Office, a prominent site and sumptuous manner that would equal or surpass its neighbours." I would definitely agree with that assessment.

I think the building with the six Corinthian columns survived while most or all of the other 1901 buildings did not. The columned building on the right in Google Street view is the Bank of Montreal, address 119 Rue Saint-Jacques. The columns match and the relief inside the pediment on both buildings match.