*Emily is an emerald green tender engine who lives and works on the Island of Sodor, and is the North Western Railway's number 12 engine.

Emily is a kind, caring, helpful and intelligent engine, who acts as a sisterly figure to other engines. She is sensitive to the feelings of others and is quick to apologise if she wrongs someone, whether knowingly or not. She will stand up for other engines if they are being teased or treated unfairly and is unafraid to speak her mind, even to authority figures.

Emily often acts as a level-headed voice of reason, particularly when the other engines jump to conclusions or when a friend needs moral support. She is also a very competent, clever, adaptable and quick-thinking engine, and as such is often relied upon for advice, a trait which eventually led her to becoming the railway's official "safety engine".

Though Emily can occasionally come off as bossy, rude or impatient, these traits are usually unintentional on her part and come from a place of concern for the well-being of others. She does, however, have occasions where she can be quite vain or overconfident, usually as a result of feeling she knows best, though once brought back down to earth she is always sure to make amends.

Since around the eighteenth series, Emily has been prone to feelings of inadequacy or jealousy, often longing for more excitement in her life when she hears of others' adventures, or for the same talents as them. Thus, making her a character that the audience can sympathise with.

Emily is based on a Great Northern Railway (GNR) 1003 Series Stirling Single 4-2-2.

Designed by Patrick Stirling in 1870 with 8ft 1in driving wheels and outside cylinders, these engines were specifically built for high-speed expresses between London and York. A total of fifty-three engines were built at Doncaster between 1870 and 1895, unofficially divided into three series based on their dimensions: the twenty-seven No. 1 Series engines between 4/1870 and 4/1880, the twenty 662 Series engines between 2/1881 and 12/1893 and the six 1003 Series engines between 11/1894 and 4/1895. George Frederick Bird labelled these series G, G2 and G3 respectively in his book The Locomotives of the Great Northern Railway, 1847-1910 (albeit with some inconsistencies). Under Henry Ivatt's classification scheme in 1900, the 1003 Series were classified A1, the 662 Series and No. 1 Series (which by now had all been rebuilt to the 662 Series' dimensions) were classified A2 and the eight engines rebuilt with a domed boiler were classified A3.

Emily is of the 1003 Series, or Class A1, which feature an updated Stirling boiler, larger cylinders and most notably an upgraded cab. The last of the Stirling Singles, No. 1006, was withdrawn on 14th February 1916.

The only surviving member of all classes, the pioneer No. 1, is on display at the National Railway Museum. The engine in preservation carried a small capacity Stirling tender, which was given in its first restoration in 1909. A derelict original Stirling Single tender used as a water and sludge carrier was later found in Peterborough and, after restoration, was paired with the engine in 2014.

Emily is painted emerald with yellow lining and brass fittings. Her valences and the back section of her running board are painted chocolate brown, while the top of the front section of her running board is grey. Her bufferbeams are red.

When Emily debuted, she had bronze buffers with black buffer shanks. In the eighth series, her buffers became silver, while her buffer shanks later became red in the eleventh series. In the twenty-third series episode "Chucklesome Trucks" and in several magazine stories, Emily's buffers were briefly black as a mistake, but returned to being silver in later episodes.

From the twenty-fourth series episode "Emily to the Rescue" onwards, Emily has her number 12 painted on the middle square on the sides of her tender in beige with chocolate brown borders.*