All the London railway stations :
Brixton is on the Victoria underground, and also the South Eastern rail network.
The Victoria underground line is accessible, with lifts between street and platform, and the step / gap onto trains is reasonable. See pictures and further accessible transport detail from links below.
The National Rail station is entirely NOT accessible to wheelchair users with steps to all platforms, no staff available and no ramps available.
Underground lines : Victoria, National Rail South Eastern.
Accessibility friendly : Yes. (A step 43 - 44 mm/ gap 74 - 76 mm) on Victoria line platform.
Lift access between street and platform for the underground station only.
More comment :
National Rail Map & Guide,
Inclusive Britain Station Guide
Victoria line only.