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A bit out of date, but this interactive map of step free London is very good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 :
Bethnal Green is a station on the Central underground line, and also the Greater Anglia rail network. The central line station is not accessible to wheelchair access.
There is an escalator and then an additional 26 or 28 steps.
The National rail station also has 39 stairs.
Underground lines : Central, National Rail Greater Anglia
AccessAble Access Guide :
Accessibility friendly : Central - No. National Rail - There are only 39 steps, so I do not think so.
General description : See wiki
More comment : National Rail maps & guide