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A bit out of date, but this interactive map of step free London is very good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 :
Brimsdown is on the Greater Anglia rail network. The platforms are accessible via fairly steep ramps, and there are no boarding ramps onto trains. See pictures and further accessible transport detail from links below.
National Rail : National Rail Greater Anglia
AccessAble Access Guide :
Accessibility friendly : This station appear to be fully accessible. I am not so sure that it is after studying the guides.
General description : See wiki
More comment : National Rail map & guide