Board Thread:General Discussion/@comment-45931498-20200528185654/@comment-25364614-20200529002400

Daihaseisai Arc is completed in the anime in both Index and Railgun.

Personally I'd recommend not watching things in chronological order but production order (Index, Railgun, Index II, Movie, Railgun S, Index III, Accelerator, Railgun T). But whatever suits you.