Unsourced secondary material online (e.g. https://en.wikipedia.org/wiki/George_Walter_(1790–1854) [accessed 28/12/2017]) suggests that the Robert Johnston who was co-promoter of the early Southampton railway scheme was Robert Johnston (1783-1839) of Jamaica.
Precursor of what became the London & Southwest Railway. Robert Johnson and Abel Rous Dottin reportedly issued a prospectus in 1830, and the scheme was revised in 1831 at Dottin's house in Southampton.
Sam Fay, A Royal Road: Being the History of the London & South Western Railway, from 1825 to the Present (1882)