I heard both terms used in an episode of 'Sherlock'. It seems like one term or the other should do for both surface and underground trains.
Learn English – English subways have ‘Cars’, but English Surface Trains have ‘Carriages’. Why the Difference
british-english
Related Solutions
You are correct that Filet is the French spelling of fillet. According to the Oxford online dictionary:
French spelling of fillet, used especially in the names of French or French-sounding dishes
filet de boeuf
a fleshy boneless piece of meat from near the loins or the ribs of an animal:
a chicken breast fillet
[mass noun] :
roast fillet of lamb (also fillet steak)
a beef steak cut from the lower part of a sirloin.
a boned side of a fish.
So, when preparing a dish, as an ingredient, you should use the word fillet. When referring to the name of the dish, it will depend (as you note: filet mignon).
Additionally, the Cambridge online dictionary states that filet is the US spelling of fillet:
US for fillet
The country of which I am a citizen is the United Kingdom of Great Britain and Northern Ireland. Great Britain is the largest of the British Isles and is home to England, Scotland and Wales. I was born in England and, apart from several extended periods abroad, have lived my life in England. That makes me ethnically English and politically British. Although Great Britain is a geographical term, British describes nationals of the whole of the United Kingdom and Britain is sometimes used to mean the United Kingdom. Things are often perceived differently abroad, and even by some of the British themselves. The whole political entity is frequently referred to as England, even though England is only a part of it. That doesn’t usually bother the English, but it might bother the Welsh, the Scots and the Northern Irish. The best policy is to call the country the United Kingdom or, less formally, Britain. Call the people British, unless you know them to be English, Welsh, Scottish or Northern Irish or something else.
EDIT:
The title ‘The United Kingdom of Great Britain and Northern Ireland’ raises an interesting linguistic, as well as political, point. Syntactically, it’s ambiguous. Is it ‘(The United Kingdom) of (Great Britain and Northern Ireland’) or is it ‘(The United Kingdom of Great Britain) and (Northern Ireland)’?
In ‘The Isles: A History’, Norman Davies traces the various titles by which the isles have been known. From 1660 to 1707 it was ‘The Kingdom of England and Wales’. The union with Scotland in 1707 gave us ‘The united Kingdom of Great Britain’. Meanwhile, there was a Kingdom of Ireland from 1660 to 1800. In 1801, Ireland was included in ‘The United Kingdom of Great Britain and Ireland’. That state of affairs lasted until 1922 when Ireland divided, allowing the six northern Irish counties to become part of ‘The United Kingdom of Great Britain and Northern Ireland’.
The original ‘united kingdom’ of 1707 was so called because it united England (with Wales) and Scotland. The addition of ‘Ireland’ in 1801 and of ‘Northern Ireland’ in 1922 can therefore be seen as mere accretions to an already united kingdom. However, the grammatical ambiguity allows the alternative interpretation of all components of the State being united under a single crown. A good example, perhaps, of Engli-, sorry, British, compromise.
Best Answer
I think this was explained in the episode of Sherlock. The terminology depends on who supplied the original carriages or coaches. In the case of the London Underground, it was the American company (American Car and Foundry), and the American term stuck with people employed in the London Underground. That is also why British trains have 'buffet cars'. Members of the public, however, are likely to use terminology from the railways and talk about carriages or coaches.