I thought to include metro stations mostly because of the nature of the original French subreddit (https://www.reddit.com/r/MetroFrance/comments/1hcifh1/pas_vu...). It perhaps doesn't make much sense in hindsight, but I also filtered out metro stations later on in the post and it didn't change much.

I'm starting work on a follow-up which will sample more cities, though I still plan to use walking distances w network analysis for now.