You're probably thinking of the Dockland's Light Railway. https://en.wikipedia.org/wiki/Docklands_Light_Railway https://en.wikipedia.org/wiki/Docklands_Light_R

You're probably thinking of the Dockland's Light Railway. https://en.wikipedia.org/wiki/Docklands_Light_Railway https://en.wikipedia.org/wiki/Docklands_Light_Railway It's not part of the Underground, but it's also managed by Transport for London as part of its integrated network of public transport. https://en.wikipedia.org/wiki/Transport_for_London https://en.wikipedia.org/wiki/Transport_for_London