This is '6100' class 2-6-2T No. 6123 (of Slough, built 10/31 and withdrawn from Oxford in 4/62), being one of the scores of locomotives that dominated the Paddington suburban services for thirty years from 1930.
Peter Kerslake says "Freshly overhauled at the adjacent Works 6123 appears to be without a shed plate but may be heading for Reading where it was certainly allocated a few years later in 1956 when there were fourteen of the class there, an equal number to those at Old Oak Common. Introduced between 1931 and 1935 the seventy members of the class were a development of the earlier "3100" and "5100" classes and were intended for duties withn the London suburban districts working from Paddington, a number being enhanced from 1957 by having a fully lined out green livery. Whilst, as would be expected, the majority of the class were based within depots close to London a few were also at Newport, Tyseley and Bristol Bath Road in the mid 'fifties.
6100 was the first to be condemned in 1958, whilst the last nine survived until 1965 with 6106, one of the last survivors and which was withdrawn from Oxford in 1965 thankfully now preserved at Didcot, home of the Great Western Society."