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

occasionally, in contemporary colloquial usage, this is taken to refer to a toot made from a train, more generally, or from a bus line which travels at least partially underground....

@elomatreb https://mastodon.mit.edu/media/sfXg0M6vravLmqKFld4