It has been called the most famous photograph in Canadian history. It depicts Donald Smith, president of the Canadian Pacific Railway, driving the last spike at Craigellachie, BC, near Revelstoke, surrounded by top-hatted dignitaries and some of the navvies who'd been up all night laying the final track. The date was 125 years ago today, 9:22 on the morning of Nov. 7, 1885.
The photograph (courtesy of the BC Archives E-02200) was snapped by Alexander J. Ross, a photographer from Calgary. Pierre Berton, in his history of the railway, describes Ross as a hunchback but not very much else is known about him. He apparently left photography for ranching, then died in 1894. (An essay on photography and the CPR is here.)
There were actually two photos taken. On the first try, Smith bent the spike and a second had to be substituted and driven cleanly. Smith received the original bent spike and later had pieces of it fashioned into brooches for the wives of company officials.
The following day, Nov. 8, the train which had brought the dignitaries from the East continued across the completed track down to the coast to Port Moody, then the Pacific terminus of the transcontinental line. From there they travelled by steamer to Victoria where they were wined and dined. BC Premier William Smithe had wired his congratulations to Prime Minister Macdonald on the completion of the railway: "May it prove a permanent bond of union between eastern and western provinces of this great Dominion."
The Kamloops Art Gallery hosts a highly interesting website about Andrew Onderdonk and the construction of the CPR through BC here. The best written source is Robert Turner's book, West of the Great Divide: The Canadian Pacific Railway in British Columbia, 1880-1986 (Sono Nis, 2nd ed. 2003).