Rolling stock used on the London Underground and its constituent companies has been classified using a number of schemes.
The numbering and classification of electric multiple unit stock on the London Underground is usually related to the type of line that the trains are used on in the central area of the network.
Sub-surface lines were built using the cut-and-cover tunnelling method and use trains of similar size to those on the National Rail network.
[a] The method of classification depends on the type of line the train operates on: Tube Stock is classified by the anticipated year of delivery at the time of ordering.
When the LPTB was formed, the ex-Metropolitan Railway units were incorporated into this series (types MV, MW, T, V, VT and W).