Plan of Euston railway station as it was in 1888.
The earliest part of the station (dating to its opening in 1837) are the four tracks to the east (right) of the Departures platform to the right of the hall, and the famous Doric arch, located over what is marked in the plan as the Entrance to the courtyard.
The Great Hall and western departure platform were added in 1849, and the further arrivals platforms in the 1870s.
Additional platforms were created in the 1890s with a major expansion to the west (left) of the station, and by filling in one of the carriage sidings on the arrivals side of the station to create a further two platform faces.

The plan can be compared with a plan of Euston in 1938, and an aerial view from 1936. (Both copyright-expired in the UK (unknown creator: publication+70), but still copyright in the United States until the 2030s (URAA restoration, corporate work: publication + 95), so not uploadable to Wikipedia). Different phases of the station's evolution are marked by different types of roof-line.

A detailed history is given in the Survey of London coverage (1949), vol. 21, pp 107–114