In 1840 James Mitchell left Kerr, Mitchell & Neilson and the company was renamed Kerr, Neilson & Co. Walter Neilson took the locomotive building business further, wanting to build Scottish locomotives for Scottish railway companies who until then had to go to England for their locomotives. By 1843 locomotive construction had begun. James Mitchell returned to Glasgow and ran the financial side of the business as a partner and the company became known as Neilson & Mitchell once more.