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A bit out of date, but this interactive map of step free London is very good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 :
Dagenham Dock is on the C2C line, and is partly step-free to one platform. See pictures and further accessible transport in London detail from links below.
National Rail lines : The Station is on the C2C line from Fenchurch Street towards Southend, via Barking and Rainham.
Accessibility friendly : Check closer before you use this station.
General description : See wiki
More comment : National Rail Map & Guide, the station has 2 disabled parking spaces. It is not entirely clear how both platforms are accessible.