You get to the Track Access page via these menu options on the new website:
Industry & commercial > Information for operators > Sale of access rights > View consultations on track access contracts and supplementals
At the moment this takes you to the track access page on the old website
http://archive.nr.co.uk/browseDirectory.aspx?dir=\Track Access&root=
but it will migrate to the new website in due course as there is a note that says:
"Please note: information within this section is continuing to be updated before future migration to the new website at
www.networkrail.co.uk, and will be available here until May/June 2018."
The Sectional Appendix downloads are at:
Industry & commercial > Information for operators > National Electronic Sectional Appendix