In 1868 John Aspinall became a pupil of John Ramsbottom, and later F. W. Webb, in the locomotive works of the London and North Western Railway at Crewe. Once he had completed his pupillage, in 1872, he was sent to study the developments that were taking place in the USA in the fields of locomotive and steel making. On his return he became assistant manager of the steelworks at Crewe and then, in 1875, he moved to the Great Southern and Western Railway of Ireland as the works manager and assistant locomotive superintendent at Inchicore, near Dublin.

In 1886 Aspinall was invited to become the chief mechanical engineer of the Lancashire and Yorkshire Railway, for whom he completed, equipped, and organized the engineering workshops in Horwich. In 1899 the company took the unusual step of appointing him general manager, a tribute to his administrative as well as engineering abilities. In 1904 he introduced one of the first main-line electrification shcemes in the country when he initiated the instalation of electric traction on suburban railway lines such as Liverpool to Southport.

For his work at Horwich in the First World War Aspinall was knighted in 1917. He retired from his position as general manager in 1919 and was appointed a director of the company, but resigned later in the same year on his appointment as consulting mechanical engineer to the newly formed Ministry of Transport, a position he held until 1927.