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A bit out of date, but this interactive map of step free London is very good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 :
Carshalton is on ThamesLink, and is only accessible to wheelchair users on platform 2. See pictures and further accessible transport in London detail from links below.
National Rail : ThamesLink
Accessibility friendly : There is step free access to platform 2 via side gate (to Sutton) only. Subway with steps to platform 1 (to London). This station is therefore not accessible. Nearest step-free station is Sutton.
General description : See wiki
More comment : National Rail Map & Guide.