There are some restrictions for melds in the Metrum Classic variant, but no restrictions in the Metrum Classic Beginner variant.
All standard melds in the Metrum Classic variant must contain at least one single-line station card.
In the example below, the meld on the left consists of Elephant & Castle, Embankment and Baker Street but it is invalid because there is no single-line station as part of the meld and therefore cannot be played. The meld on the right which contains Marylebone in place of Baker Street is valid.
If a player wants to play two or more multi-line melds, then there cannot be any lines duplicated across their melds.
In the example below, both multi-line melds are valid on their own, but cannot be played together by the same player because the Stamford Brook and Sloane Square (partially hidden) cards are both on the District line. In the London Base game, either card could be replaced by a single-line station on the Piccadilly or Victoria lines such as Heathrow Terminal 5 or Pimlico.