Laura Flora posted on the group called Shit London - a group posting photos of "unintentional human comedy" in the city - asking for advice.
She wrote: "I need your help! I have a 23-year-old autistic son who is obsessed with trains.
"Currently his obsession is the underground and how many platforms there are at each station. And I mean each station."
Flora added: "I would really like to not have to spend the next three months of my life getting on and off trains counting platforms, so how about if you are at a tube station you could count the platforms if you can see them and tell me which line you are on and north/south/east or westbound?
"I would like to buy a massive wall poster and be able to put notes at each and every station.
"Unless someone works for the underground and knows that a list exists somewhere with this information?"
Flora accompanied the post with a photo of the entrance of a tube flooded in spring in light of the group's purpose.
Lo and behold, Londoners came to her rescue, with nearly 700 comments detailing the number of platforms at people's nearest tube stations.
Some comments came with impeccable detail:
"Tufnell Park on the northern line has two platforms. The station is currently closed to replace the old lifts. Will re-open in March 2016. In the meantime we can get the 134 bus for free to take us to Kentish Town and vice versa."
"Don't know if anyone's done Morden - it had five platforms, all northbound, but you can't get to platform one, it'd be closed off. And as there is only one rail between platforms three and four, they lead onto the same train."
After the hundreds of comments came in, people began asking which ones were missing so they could help her out.
Flora said she was "compiling the list" and was going to get back to everyone.
One person added: "I can't really help right now 'cause I won't be in London anytime soon. I just want to say how all of this is blowing me away. Y'all must be proud.
"And Laura too. Your effort for your son is beyond comprehension."
To add your pearls of wisdom about tube stations near you, visit the thread.