While on the GSWR, he selected Kilmarnock as the site for the new locomotive works. The first engines to be constructed ther were his 2 class 2-2-2s. From 1860 he adopted the domeless boiler, also used by his brother James, and son Matthew, on the GSWR and South Eastern Railway, and Hull & Barnsley Railways respectively.

His locomotive design innovations included:

• GNR 2-4-0s, then 2-2-2s in 1868, 0-6-0s, 0-4-2s and 0-4-4T engines
  
• His famous 8 foot driving wheel 4-2-2s built in 1870-95. These excellent performers, ran some of the fastest passenger trains in the world during the 1870s.

However, he was becoming out of touch with modern railway design practise and by the end of his term in office was hostile to the idea of coupled express engines and bogie passenger stock.