Dominion Express seems to have been in the freight/courier business. Founded 1873, independent until 1882 when they were bought by Canadian Pacific Railway. Operated as Canadian Pacific Express from 1926. Later merged with Smith Transport, then converted to Interlink Transport about 1994 and seems to have ceased operation about 1997.
Canadian Pacific’s Dominion Express had a building put up on the northwest corner of Simcoe and Wellington streets (Toronto) in 1914. Given that the OP is Toronto based this might be the source building. They also built in Montreal about the same time. Dominion Express probably had buildings in other major Canadian cities as well as offices in many train stations (particularly those served by CP?).
Sources:
http://www.dardpi.ca/wiki/index.php?...xpress_Company
http://www.trha.ca/trha/exploring-th...-express-sign/
https://spectrum.library.concordia.ca/492/
http://www.trainweb.org/oldtimetrain...s/history.html
http://www.imtl.org/montreal/buildin...on_Express.php