Mastodon 101: A "Subtoot" is a toot written by someone currently travelling in a subway. They are usually used to praise the effects of good public transit infrastructure.

𝘛𝘩𝘦 𝘮𝘰𝘳𝘦 𝘺𝘰𝘶 𝘬𝘯𝘰𝘸