The original 2-series single decks, are they still single deck? Exactly how many single deck routes can be double decked? Is it an efficient use of resources to do so? This title is pretty long isn't it? I've taken inspiration from light novels.

London's debatably deckable single deck bus population.
Blood red city roads.

Even if you aren't well versed in the numbering system you would've noticed how majority of routes in Central London are numbered below 100, with the predecessor being a short mutation of current 366 between Jenkins Lane and Barking, but via Gascoigne Road.

Either way, I explore, mind you with calculations that are invalid from the start due to different conditions (i.e not fair tests), to come up with the answer to questions on the line of: "Is it cost-effective to convert single deck routes to double deck?" With the mindset of "if it can take them [double decks], convert them." For now though...

The likes of the 1-series being dominated but not excluding Central routes of old, take 133 and 113... which is a renumber which we'll get to.

The 2-series served one solid purpose, a solid rule. Single deck routes, some survive with restrictions, 227 (formerly 109) at Shortlands Station, 236, and Reedham Station which had 234 but through natural selection of survival of the strongest link sees 455 now, in future 312 again.

The 3-series mainly confined to Country routes north of the river, 313 and 370.

The 4-series mainly confined to Country routes south of the river, 401, 402, 465. I mention 402 but a shame Arriva pulled it back from Bromley. There is an exception as well, current White Bus' 441 used to reach Herfordshire, even if it was close to London's boundaries.

The 143 paradox. Squires Lane the anti-Trident.

For the most part besides the special needs routes, London was double deck city. There was positives in that aspect of flexibility but the biggest downside of all was that coverage was low as a result, mainly in areas that have through services but have some nimby or likening to not have curtains or some other reasons, Dulwich Village, Elmstead Woods. Coincidentally the very areas where the first one-person operated routes, also trialling with flat fares had new routes in the late 60s, P4 and B1 respectively. The latter is a neat half of current 314. Whilst Gospel Oak was the restriction for C11, the W9 didn't see much of an upgrade unlike the rest.

Beckenham. The source of 126, 162, 229, 254.

- 209 was the previous number of 124
- 214 was the previous number of 113 (Mill Hill Broadway - Hendon Central), no Sunday service
- 229 (previously 609) was Penge to Bromley via Westmoreland Road as per today's N3 with a PVR of 4 single decks (highest at 7 on Saturdays) and one double, renumbered 254 a year later keeping it's one double deck between Beckenham Junction and Bromley before being double decked, gaining the number 126... as you can tell from it's current routeing the 126 barely goes Bromley South. Just switch the last two digits and you have it's successor... the 8.9m E200 allocated 162 (Beckenham Junction - Eltham Station)

And I have already presented you the sheer amount of double deckable single deck routes at the top of the post in one London map form (to 66% scale), had to exclude the likes of 80, 313 and 428 as they were announced/converted in the mean time. Quick rundown however, the count is: 87.

North East map
North West map
South East map
South West map

One of the highest concentrations of red.

For more on the 2-series routes and how I came up with this count, the spreadsheet is here. As for the first question imposed in the title, off the top of my head you only have 124(209), 126/162(229), 216, 227, 236 and 251. I'm not bothered to confirm it after the hassle of researching literally every single deck route and calculating the whole process. Using London Bus Routes by Ian Armstrong as my primary source of course.

On average, above 130% seating capacity increase going from single decks to double decks, penalising an extra 19% of fuel, if in the same conditions an E400 smart hybrid is efficient to 7.3 miles per gallon whilst the diesel single deck E200 MMC is 9.1 mpg. You'll read more as you scroll right on the spreadsheet. 

   Total capacity increase ranges from 45%-58% by TfL's outdated capacity standards. I would've calculated (not really as I'm cutting this short) if it's worth converting certain routes based on their patronage, dividing their PVR and vehicle capacity. The 481 trumping itself as the 2nd least used double deck route makes me motivated not to bother.

Before I end this here, where you'd usually here "like, subscribe" I'm here thinking "if you could share this around, that'd be nice" as I didn't expect working a month on this. The more eyes on it the more awareness. Perhaps a little eye opening.

Okay forget the excuses. So yeah, stay safe.

Fun facts: I just wanted to mention 54 met almost every digit of a x54 route. 

The previous 154 was just today's 54 between Elmers End Green and Plumstead Common but using Shooters Hill and presumably Charlton Park Lane and Hornfair Road. 

   The 54 met the next 154 at West Croydon, this very 154 having a scheduled 254 at some point (West Croydon - Beddington Lane - Mitcham - Wandle Road - Morden) which only lasted 17 months. 

   The 354 was as today but renumbered to 351, allowing for Metrobus to reuse the 354 number for what is now the 654 which was a daily single deck route terminating at a previous terminus of 54, Selsdon Farley Road. Though this 354 did touch 54 at Croydon. 

   The current 354 also touching 54 in Beckenham.