'Geeky' brothers are fundraising to send their cousin's poorly children on a luxury holiday by attempting to visit all 270 London Underground stations in 24 hours.
Richard and Michael Brownnutt, who grew up in Wimbledon, set off from Wimbledon Station at 4.53am this morning and hope to arrive in Morden at 12.30am tomorrow morning.
They are hoping to raise enough money to send Ellie Mae, five, and Caleb, three, on a trip to see the London sights after they were both diagnosed with Late Infantile Batten Disease.
The rare genetic nervous disorder will see the siblings, from Yorkshire, eventually lose their mobility, speech and eyesight.
Richard Brownnutt, a project manager who lives in Sutton, said: "It's very rare for anyone with the disease to reach their teenage years so we wanted to get them a nice hotel while they are still around to see the sites."
He will be attempting the challenge with his brother, Michael, a 36-year-old store planner who lives in Morden.
So far they have raised more than £2,000 and Mr Brownnutt, 39, said any extra money will go towards the Batten Disease Family Association and healthcare support for the family.
Caleb, three and Ellie Mae, five
Asked where the idea for the challenge came from, he said: "My brother and I have a bit of a geeky side. We just like challenges like this and my grandfather who is also my cousin's grandfather worked on the railway in London.
"We are both from Wimbledon originally and use the Underground to get to work but there are so many parts that we haven't travelled on and haven't seen.
"Then we started to timetable what is possible and we worked out that you can just about scrape it all in one day if you are lucky.
"It's also been a mathematical challenge about mapping. I've got a mathematics degree so that's where that came in."
The brothers will be wearing orange t-shirts with pictures of their little cousins and said they will keep themselves busy by talking to fellow commuters and tweeting about their challenge using the hashtag #tube270.
They hope to optimise their performance using apps Tubetracker, which tracks live departure times and Stationmaster, which tells users the best door to leave a train from.
To support their cause, donate via their Justgiving page.
Keep track of their progress throughout the day by visiting their Twitter pages @brownnutt and @mikebrownnutt.