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A bit out of date, but this interactive map of step free London is very good
All the London railway stations :
Underground lines : London Overground (North London line)
Accessibility friendly : Partial.
Step free access platform interchange requires a 1300m journey via street.
Use the Kingswood Terrace entrance for the westbound platform.
Use the Palmerston Road entrance for the eastbound platform.
General description : See wiki
More comment : National Rail Map & Guide, Platforms are accessible from the street via ramps. Exit of the station is necessary to change platforms.