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My thinking is that early May or early September would be best, as it would hopefully be warm enough outside in the early morning and late evening, but not swelteringly hot in the subterranean parts of the network.

What do those who've actually tried the full challenge think?

historically its always been May.

I'd say that comfort is secondary. However, climate factors do affect reliability and, I suppose, crowding in passageways.

April, may ar October I think

Doesn't make too much difference i think. although there is a winter timetable, and a very cold start/finish and less daylight in winter.

Leaf fall timetables add a minute or two to journeys beyond Rickmansworth, according to TFL. I think that's the 1st of October to the 2nd of January.

I don't think it matters really. I suppose in school holidays there'll be more passengers during the day, and a kid could pull an emergency stop lever or something like that.

Since they've introduced the free transport on buses for kids, I theorise that all routes should be designed so that you are using no buses between 3.00 - 4.30, or should attempt only in the school holidays!

But during the school holidays, wouldn't the kids just be using the buses twice as much, but not in a good manner if you catch my drift....

Hmm... in my experience, kids would tend to be more spaced out during the school holidays so you don't get a massive crush of them all at once which can really **** you over.