Captain William Scarth Moorsom (1804–1863) was an English soldier and engineer. He was born in Whitby to a military family, being the son of an admiral, and trained at the Royal Military College, Sandhurst, becoming a captain in the 52nd regiment. After assisting Robert Stephenson he created railway lines in England, Belgium, Germany and Ceylon.
Moorsom was the youngest of the four sons of Admiral Sir Robert Moorsom, who had served at the Trafalgar, and his wife Eleanor.
He entered the Royal Military College in 1819, and became especially adept in fortification and military surveying. In 1823 he joined the 79th Highlanders Regiment, then stationed in Ireland. During his stay there, he made a survey of Dublin and its neighbourhood. This remained in use until superseded by the Ordnance Survey publication. In 1825 he served in the Mediterranean at the rank of lieutenant of the 7th Fusiliers. In 1826 he transferred to the 69th Regiment, and then to the 52nd Light Infantry in Nova Scotia.
During this time he served as deputy quartermaster-general. He produced a survey of the harbour and environs of Halifax, along with reports on transport feasibility to all parts of the province, and published a monograph Letters From Nova Scotia; comprising Sketches of a Young Country in 1830.
Although he was highly regarded he was unable to purchase a suitable promotion so returned to England and bought out his commission in 1832. He had met his wife, Isabella Ann Morris, daughter of Lewis Wilkins, judge and head of the supreme court in Nova Scotia. They lived with his father at Cosgrove Priory, near Stony Stratford, until his death in April 1835.
With his experience of military surveying, Moorsom assisted in the construction of the London and Birmingham Railway construction of which had begun in 1833 and of which his eldest brother, Constantine Richard Moorsom was Secretary to the Board. Moorsom's survey of the valley of the Ouse, which allowed for the line to be straightened, and a large amount of embankment dispensed with, attracted the attention Robert Stephenson He then spent two years studying new railway lines all over the country and in 1836 was approached to undertake a survey of the country between Birmingham and Gloucester for the purposes of building a railway. The Birmingham and Gloucester Railway had found Brunel's proposals out of its financial reach. Moorsom's brief in 1836 was to build the line as cheaply as possible, which he did by following open country, avoiding populated areas where land prices would be higher. Arriving at the Lickey Hills there was no option but to climb them, using cable assistance if necessary.