In the Metrum Classic variant, once a player has played their first meld to the table, then they have some advanced play options that they can make.
Instead of drawing a card from the face-up selection cards, once a player has played a meld to the table, they can switch one card from their hand with any played meld (either their own or another players), as long as they leave the meld intact. It is possible to pick up an Inspector card from an existing meld.
In the example below, player 2 has a Northern Line meld, consisting of Kings Cross, Camden Town and London Bridge. Player 1 has a Central Line meld, and has 3 cards in their hand including Tooting Broadway on the Northern Line, and two stations on the Hammersmith & City Line (Goldhawk Road and Barking). They can choose to swap Tooting Broadway from their hand with Kings Cross on the table, then can play Kings Cross as part of a Hammersmith & City Line meld along with Goldhawk Road and Barking.
Adding to other players’ melds
Players can also add to other players’ melds, which can be a useful tactic to allow a player to go out.
In the same example above player 1 could draw Aldgate East from the Selection Cards and play it in a Hammersmith & City Line meld along with Goldhawk Road and Barking. This would leave them with just Tooting Broadway in their hand, which they could add to player 2’s Northern Line meld as it only has 3 cards and standard melds can have either 3 or 4 cards. The Tooting Broadway card would count as part of player 2’s score, but would enable player 1 to go out and get the going out bonus.