Small pack of promo cards for Next Station: London. The pack is called "Open Day" and consists of 4 round Open Day cards. The promo pack was handed out during Essen Spiel 2022. Later available in Pack Bard from Dice Tower 2023 Kickstarter campaign.
The official rules:
"The controller of the first round shuffles the 4 Open Day cards face down. They draw one at random and return the other 3 back to the game box. This card shows all players the metro station where the Open Day is located. All players mark this station on their game sheet by drawing a Tourist Site symbol around the station located in the centre of the specified district. This station becomes a 6th tourist site through which players can run their metro lines in the game. As in the basic rules, count the number of tourist sites which each of your metro lines passes through, also considering the 6th tourist site of the city indicated by the Open Day card."