They're the only Tube stations not to contain any of the letters in LEYTR.
You've probably heard a pub quiz question along this line for St. John's Wood station, which contains no letters of the word 'mackerel'.
What makes Chigwell so unique? It's the only Tube station name not to contain any letters in the word 'transport'; Borough and Bow Church are the only stations to contain no letters from the word 'Leyland'; and Archway solely contains no letters from the name 'Ken Livingstone'. Aldgate and Temple stations are 'Boris Johnson'-less.
Have we spent hours devouring Tube station names? No. We've used a cracking, yet devastatingly simple, online lexicon - the Tubewhack. Fans of Dave Gorman will have surely heard of his Googlewhack Adventure, where he attempted to find two unrelated and random words that, when typed into Google's search box, provided one solitary web page. This is called a Googlewhack.
You have to be careful, though, since immediately as you blog about the Googlewhack you've identified, it no longer can be classed as such as your blog then becomes the second result associated with the two words.
Tubewhacking is based very heavily on the Googlewhack principle, though when one is identified, it is fundamentally safe. On the Tubewhacking website, just type the most random single word in the box and see how many Tube station names are shown. This identifies those without those letters in their name(s). With just one station remaining, you've got yourself a Tubewhack.
National's Tubewhack is Shepherd's Bush; Adonis' is Temple; Omnibuses' is Archway; Stagecoach's is Kilburn; Competition's is Vauxhall; Pendolino's is Archway and Trolleybus' is Wapping.
You can get more adventurous, applying weird and wonderful criteria: Ruislip, for example, is the only station not to contain any of the first 8 (abcdefgh) letters of the alphabet; Balham is the only Tube station not to contain any letters from the word Underground - it's also the only station to not contain any of the last 13 (nopqrstuvwxyz) letters of the alphabet; and Upney is the only station not to contain any letters from one of your Editor's name....