The London Assembly has begun a survey to map a picture of overcrowding hotspots on rail routes into the city.
Its transport committee wants to identify stations where passengers have to wait for several full trains to pass before boarding, for example.
Almost 700,000 people commute via the 50 overground rail routes into London.
Last month the National Audit Office said rail passengers in England and Wales face more crowding and fare rises unless the network is expanded.
Overground rail routes account for about a fifth of all commuting in London.
The London Assembly investigation will focus on the passenger experience as well as mapping and analysis of how effectively overcrowding is currently being tackled.
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