Metrum is a meld making card game in a similar vein to Rummy. In Rummy the aim is to collect sets of cards (called melds) of either the same face value or a run of consecutive values in the same suit and to play them onto the table to score points. Rummy is played with a standard pack of 52 cards with suits of clubs, diamonds, hearts and spades (some variants of Rummy are played with multiple packs).
In contrast a card in Metrum consists of a single metro station and any metro lines that run through that station. Melds in Metrum mostly consist of a set of station cards with the same metro line. Connection stations have multiple lines and therefore there is more flexibility about which meld a connection station could belong to (although they can only belong to one meld).