Complex sentence: When complete, Platform 1 will be the Up Main, Platform 3 will be the Down Main with Platform 2 as a bidirectional through platform although the main function of platform 2 will however be to serve down line trains.
Split into simpler sentences: When complete, Platform 1 will be the Up Main, Platform 3 will be the Down Main with Platform 2 as a bidirectional through platform. The main function of platform 2 will however be to serve down line trains.