The CNR started it's life in January 1923. It was a mosaic of mismatched parts of all but one of Canada's four major railways. 
It was comprised of the Grand Trunk Railway (GTR),  Intercolonial Railway (ICR) 
and the Canadian Northern (CNoR).
The locomotives shown on this page were in some cases inherited from the previous railways and in other's bought by the CNR. The CNR was in need of a lot of new equipment after the merger to enable it to compete with the one remaining private railway, 
the Canadian Pacific Railway (CPR).
The rivalry between these two railways continues to this day.
The CNR had the disadvantage of being government controlled and had to support many unprofitable line because of political interference.
This cost a great deal of money which a certain portion was written off by the government each year but the CNR usually showed an annual loss because of it.
Those losses plus a staggering debt load that was inherited from the merger left the CNR in the position of having to ask the government for hand outs each year.

These steam locomotives were operated until the process to replace them with diesel electric engines began in 1950. This was brought on by a serious coal strike in the US that caught CNR with very low reserves of coal and cause the temporary shutdown of some services.
This gave the government the incentive to provide the necessary funding for the conversation.
The process started on the east and west coasts of the country, working towards central Canada.
The first passenger train to be converted was the Ocean Limited, which ran between Montreal and Halifax in 1954. The process was completed by the mid 1950's.

There was a fair amount of opposition from within the CNR to this change but it was necessary because of the lower operating costs of the diesel electric engines.
They didn't necessarily run any faster but could go much further distances between refueling and required service.
 

For all you steam fans, this page is a must