During his time with the Bank of Montreal, Angus was free to pursue opportunities for private investment. In 1868, he went into partnership with the future Lords
Mount Stephen and
Strathcona at the time that they were becoming interested in developing railways across to the
Canadian West. Their ventures were largely financed by the Bank of Montreal, of which Mount Stephen was president, and as his number two at the bank, Angus was closely involved. Angus resigned from the bank in 1879, briefly relocating to
St. Paul, Minnesota, to represent the group's interests there as vice-president of the
Saint Paul and Pacific Railroad. In
Minnesota, Angus had worked closely with
James J. Hill, constructing and improving the line. But by 1880, he spent most of his time with Mount Stephen as they made numerous trips to
Ottawa, New York, and London to negotiate the
land grants,
subsidies, and building of the
Canadian Pacific Railway. With Angus providing the analysis and Mount Stephen the acumen, they proved to be a formidable pair. Angus was general manager of the CPR until the appointment of Sir
William Cornelius Van Horne in 1882, when Angus became vice-president. In that position, Angus was entrusted with the creation of the eastern network, notably the extension of the
Ontario and Quebec Railway and the purchase of the western section of the '
Quebec, Montreal, Ottawa and Occidental Railway' in 1882. The construction of the CPR was fraught with financial peril, testing the resilience of the syndicate –
Hill resigned in 1883, followed by Angus' close friend
Duncan McIntyre the next year. To lobby for funds more successfully, Angus resigned from the St. Paul Railway in 1884. The CPR, completed in 1885, was an immediate financial success, becoming "the world's greatest transportation system". He remained vice-president of the CPR after
Lord Mount Stephen resigned from an active role as president in 1888. Apparently never aspiring to the position, Angus supported Stephen's selection of
Van Horne as president and, 11 years later, Van Horne's choice of
Lord Shaughnessy as his successor. Angus would serve as a director and committee member of the CPR for over 40 years, necessitating frequent trips to the
Canadian Pacific Offices on
Trafalgar Square in London. ==Banking and industry==