1840 in rail transport

March events

 * 9 March – The Wilmington and Raleigh Railroad is completed from Wilmington to Weldon, North Carolina. At 161.5 miles (260 km), it is the world's longest railroad at this time.

April events

 * April – The Raleigh and Gaston Railroad is completed from Raleigh to near Weldon, North Carolina.

May events

 * 11 May – The London and South Western Railway opens its original main line throughout to Southampton (England).

July events

 * 1 July - The Midland Counties Railway of England opens its line from Derby and Nottingham via Leicester to a junction with the London and Birmingham Railway at Rugby.

August events

 * 12 August – The Glasgow, Paisley, Kilmarnock and Ayr Railway opened between Glasgow Bridge Street railway station and Ayr, the first inter-urban railroad in Scotland.
 * 17 August – The first railroad built in Milan, Italy, the Milan and Monza Rail Road opens for service.

October events

 * 8 October – Formal opening of first section of the Taff Vale Railway, the first steam-worked passenger railway in Wales, from Cardiff Docks to Navigation House (Abercynon). Public service begins the following day.

December events

 * 21 December – Manchester and Birmingham Railway in England completes Stockport Viaduct (but does not yet bring it into use). It is one of the largest brick structures in Europe.

Unknown date events

 * By July – August Borsig completes the first steam locomotive built in Germany.
 * Mordecai W. Jackson and George Mack partner to create a farm implement manufacturing company that will eventually become Jackson and Woodin Manufacturing Company, one of the constituent companies of American Car and Foundry Company.

January births

 * January 8 – William Dean, Chief Mechanical Engineer of Great Western Railway of England 1877–1902 (d. 1905).
 * January 29 – Henry H. Rogers, American financier who helped finance and build the Virginian Railway (d. 1909).

February births

 * February 7 – Samuel W. Fordyce, president of St. Louis, Arkansas and Texas Railway 1886–1889, St. Louis Southwestern Railway 1890–1898, Kansas City Southern Railway 1900 (d. 1919).

March births

 * March 31 – Benjamin Baker, British civil engineer, designer of the Forth Railway Bridge (d. 1907).

April births

 * April – William Sykes, English railway signalling engineer (d. 1917).

June births

 * June 6 – William Dudley Chipley, president of the Baltimore and Ohio Railroad 1873–1876 (d. 1897).
 * June 14 – William F. Nast, president of the Atchison, Topeka and Santa Fe Railway September 1868.
 * June 27 – Alpheus Beede Stickney, first president of Chicago Great Western Railway 1884–1909 (d. 1916).

August births

 * August 23 – Brayton Ives, president of Northern Pacific Railway 1893–1896 (d. 1914).
 * August 25 – George C. Magoun, Chairman of the Board of Directors for Atchison, Topeka and Santa Fe Railway in the late 1880s (d. 1893).

November births

 * November 17 – Edmund Morel, English-born civil engineer in Japan (d. 1871).
 * November 24 – Henry Kirke Porter, American steam locomotive builder and founder of H. K. Porter, Inc (d. 1921).

April deaths

 * April 12 – Franz Anton von Gerstner, Bohemian-born railway civil engineer (b. 1796)