What font is this?

I would hazzard a guess, that since it is connected to the District line, it is in fact an Edward Johnston - the father of modern calligraphy - whom taught at the Central School of Arts and Crafts in London's Southampton Row, where he influenced the typeface designer and sculptor Eric Gill.



He is most famous for designing the sans-serif Johnston typeface that was used throughout the London Underground system. Microsoft seem to have taken all credit for the creation of Ariel typefaces, which is a shame.



If you dig deeper, you find the myth that there once there was a typeface called Helvetica. It was extremely popular.Later came a software company called Microsoft. They “borrowed” Helvetica for their operating system and called it Arial - if that were true, they didn't even have the imagination to create an original name.



When PENGUIN books was founded in 1935 with the radical concept of producing inexpensive paperback editions of high quality books, it adopted an equally progressive approach to typography and cover design. Under Jan Tschichold in the 1940s and Germano Facetti in the 1960s, Penguin became an exemplar of book design.



Returning to London from a weekend at the Devon home of the crime writer Agatha Christie in 1934, the publisher Allen Lane scoured Exeter Station for something to read. All he could find were reprints of 19th century novels and Lane decided to found a publishing house to produce good quality paperbacks sold at sixpence each, the same price as a packet of cigarettes.



Lane’s secretary suggested Penguin as a “dignified, but flippant” name for the company and the office junior Edward Young was sent to sketch the penguins at London Zoo as its logotype. Young was then asked to design the covers of the first set of ten paperbacks to be published in summer 1935 including Andre Murois's "Ariel" and Earnest Hemmingways "A Farewell to Arms". Considering illustrated book covers to be trashy, Lane insisted on his following a simple horizontal grid for Penguin’s jackets in colours that signified the genre of each book: orange for fiction, green for crime, and blue for biography.



So the first book was Ariel, with its cover having the simple name of the author and its title printed in Eric Gill's sans serif. Eric Gill being the student of Edward Johnston...

Agreed. I think it's arial bold and than some of the letters were squished together in photoshop. Just my two cents.