All the London railway stations :
Crews Hill is on ThamesLink, and is not accessible to wheelchair users to access platforms due to stairs. See pictures and further accessible transport in London detail from links below.
National Rail : ThamesLink
AccessAble Access Guide :
Accessibility friendly : Not accessible.
General description : Unmanned station. The platforms are only accessible by stairs. The nearest station with step free access is Hertford North. See wiki
More comment : National Rail Guide & Maps