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A bit out of date, but this interactive map of step free London is very good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 :
Alexandra Palace is not an accessible public transport station for wheelchair accessibility due to the steps to platforms.
National Rail : ThamesLink
Accessibility friendly : Partially accessible station, but no access between platforms and street level. The Booking Office is accessible from the street. The platforms do not have ramps available, and access is only between platforms 1 and 2, OR platforms 3 & 4. There are 26 steps to exit platforms.
General description : See wiki
More comment : National Rail Guide & Maps