Reading Corporation Transport as was, introduced route numbers (displayed on buses) in 1964 if l remember correctly. The original numbering had to be configured not to conflict with the Thames Valley Traction Co's route numbers in the Reading area.A few -
Preston Corporation buses used to show a one or two letter route code example (not mine) on Flickr.
Reading Mainline's routes all had route letters rather than numbers - I understand this was based on Reading Corporation's historic route letter system for motor bus routes (although I don't think the route letters were ever shown on buses, and route numbers were eventually adopted and displayed some time in the 50s or 60s)
The 'Street Shuttle' mini-bus networks in at least Kettering and Wellingborough that United Counties introduced in the late 80s had route letters not numbers. (Kettering example on Flickr - not my photo or account) - interurban services retained route numbers.
Routes on the current Luton - Dunstable busway mostly have letters not numbers (at present Arriva A and Z, Centrebus B and E, Grant Palmer C and CX and Hi) - Arriva F70 and F77 are the exceptions.
Some operators used letters for express services (e.g. Lincolnshire Road Car - again, not my photo) and London Transport used letters for some special services around the Festival of Britain - example here, again not my photo) - service J evolved in to the Round London Sightseeing Tour and all its successors.
And if we're counting trams as well, Nottingham's (first generation) tram routes had letters rather than numbers.
There was a short lived P (Pedigree) run by whippet worked for 6 months in 2019Cambridge busway has A and B. Previously U, C, D, F, H, R and maybe others (X?).
E (Eureka) was partly replaced by G (Godinton). There was also an H at one stage. The use of Y to Wye was considered....Ashford has the A, AD, B, C, D and G. I think there used to be an E but I think it was removed.
From the Stagecoach South West thread, Exeter has letter only routes in the city.
What other place currently or historically have/had letter only routes
E (Eureka) was partly replaced by G (Godinton). There was also an H at one stage. The use of Y to Wye was considered....
In addition, Stagecoach South East use 'Loop', one each in Thanet and Eastbourne, and prior to its splitting into two number-suffixed variations, Canterbury had 'Uni'. 'Triangle' has since reverted back to route numbers, although what was displayed on buses was an actual triangle.
The Arriva A to the hospital and B to Park Barn, which competed with Safeguard's 4/5, do link us to another entry for this post - the incarnations of Safeguard's previous routes were themselves actually "numbered" A and B!Before they withdrew their Surrey operations, the bus war in Guildford (Arriva, Stagecoach and independent operator Safeguard) led to Arriva introducing a short-lived series of three "lettered" routes: A, B and C.
I seem to recall they withdrew their 26/27 town circular routes in favour of (what they claimed was) a "simplified network" - or something along those lines.
In reality, however, routes A, B and C were just carbon copies of (parts of) existing routes run by the other two operators, with the sole purpose of trying to steal their passengers and their revenue.
Though it wasn't just Arriva who played the game, as Safeguard also launched a "carbon copy" route to rival Arriva on their route 3, but that's going off topic...
Lancaster Corp used numbers but also used O as well!Oldham Corporation used letters for their tram routes, and once these had gone buses still used letters for the ex-tram services and numbers for ‘was never a tram’ routes. The system got slightly mixed up so that a circular service in the town used ‘4’ in one direction and ‘V’ in the other - so it seems Oldhamers were familiar with Roman numerals…
Was it letter O or a zero?Lancaster Corp used numbers but also used O as well!