The Docklands Light Railway (DLR)
Joined to the London Underground network is the Docklands Light Railway (DLR) system, which operates in southeast and east London. A system of driverless trains (although there is always some kind of operator on board!), the DLR was built to help the rundown - though now much regenerated - Docklands area of London in the late 1980s, and has since been expanded considerably.

The DLR interchanges with the Underground at Bank station (which is on the Central, Northern, Circle, District and Waterloo & City lines) and also has a Zone 1 station at Tower Gateway, which is just around the corner from Tower Hill station.

You might use the DLR to travel to/from London City airport, or to visit areas such as Greenwich (home to the National Maritime Museum and Royal Observatory) or Canary Wharf (London's second financial centre behind the City).

Fares on the DLR are the same as on the Underground. Note, however, that many stations on the DLR do not have traditional ticket barriers to get in and out, so make sure you tap your Oyster card on the pads as you enter or leave a DLR station to avoid problems and a possible fine.

Train/London Overground
An increasing part of the London train network has become part of the London Overground, on which Oyster cards are now valid. Oyster cards can also be used on some other train services within London that aren't officially part of the "London Overground" network. (The Overground network is marked by a hollow orange line on the Underground Map.)

Most train services operate outside of Zone 1, so they maybe most useful if you're trying to reach places outside of the centre of the city.

There are also train services to Heathrow, Gatwick, Stansted and Luton airports, although you'll have to buy separate tickets for these!

Trams
Trams operate in Southern parts of London, in the Wimbledon/East Croydon areas. Oyster cards are accepted - a single journey costs £1, otherwise it's £2 if you purchase a ticket.