So far, I found:

English

• https://masto.canadiancivil.com/@reece, Reece Martin from RMTransit
• https://mastodon.social/@OhTheUrbanity, OhTheUrbanity
• https://social.notjustbikes.com/@notjustbikes, NotJustBikes
• https://mstdn.social/@nerd4cities, Ray Delahanty AKA CityNerd
• https://urbanists.video/ in general seems to be a PeerTube instance that focuses on transits

German

• https://zug.network/@bahnnews_austria, BahnNews Austria
• https://mastodon.social/@derloris, derloris
• https://wien.rocks/@vcoe_mobilitaet_mit_zukunft, VCÖ