He was apprenticed in 1888 to the Great North of Scotland Railway (GNoSR) at their locomotive works at Kittybrewster, just outside Aberdeen.
[2][1] At the start of 1923, the GCR became a constituent of the London & North Eastern Railway (LNER), and in the new organisation he was appointed District Mechanical Engineer for the former GC section.
[1][5] One of his tasks was to produce a scheme for the renumbering of the former GCR and GNoSR locomotives within the block of LNER numbers which ran from 5000 to 6999.
[12] Thom was responsible for only one new design of locomotive, the large Class D 0-6-4T,[2] nine of which were built between March 1904 and December 1906 for use on the coal trains which ran from the coalfield in northern Derbyshire and Nottinghamshire to the docks at Grimsby and Immingham.
[13] They were direct predecessors of the GCR Class 1B 2-6-4T introduced in 1914, upon which Thom assisted Robinson with the design work.