Perhaps not the best day to break this to strike suffering Central Line commuters, but they may well have suspected it anyway...
There wasn't a single weekday last month when the Piccadilly and Central Lines had the required amount of trains during peak evening times.
The daily service requirement at peak time for the Piccadilly Line is 79 trains for Monday to Friday. For the Central Line it's 78 trains.
Yet for the majority of December, the Piccadilly Line ran considerably under that level - as low as 52 trains earlier in the month, improving to the low 70s by the end.
The Central Line ran fewer than the required peak time trains throughout December too - running 61 trains in the period after Christmas, and around 70 for much of the month.
Now the Piccadilly Line suffered from pesky repair work, with around half the fleet of trains taken out of service at the tail-end of November. The reason being worse weather than usual and hefty leaf fall, causing the tracks to become slippery and the wheels of the trains to lock when braking, damaging both wheels and tracks.
But while they've been repaired for now, the problem could well happen again if we get particularly bad weather this year. TfL has said there's nothing ahead of time that can be done to fix that, though this year's hit was "unprecedented".
A TfL spokesperson said: "The Piccadilly Line experienced an unprecedented level of wheel flats, caused by low adhesion between the rails and train wheels when braking, which led to around half the fleet being taken out of service for safety-critical repairs."
And what's the reason for the Central Line's poor performance?
It's partly due to the age of the trains (on both the Piccadilly and Central Lines). TfL said it rarely has such problems with its newer lines.
The TfL spokesperson said: “There were also issues with electrical systems on Central Line trains, which affected the lighting and also the motors that drive the trains. These problems have not been experienced to the same extent in January.”
The mayor revealed the figures in response to a question from London Assembly member Caroline Pidgeon.
She said: "When Tube passenger figures are at a record level it is simply shameful that there were such severe shortages of trains on two Tube lines."
Pidgeon warned that:
London simply can’t afford to have such vital parts of its transport infrastructure operating at under capacity.
The failure to continually provide an adequate service is unacceptable and an insult to every commuter facing the daily grind of packed platforms and overcrowded trains.
As part of TfL's Tube modernisation plans, both the Piccadilly and Central Lines will be among those undergoing "a comprehensive renewal" and getting new trains.
Sadiq Khan has said the Piccadilly Line trains will hit the tracks from 2023.