Would add about 30 minutes to the round trip time of Thornlie trains, if extended to Cockburn Central. So the equivalent of 4 more A sets would be needed to support existing service frequencies.
And of course there is the problem of slotting 4 extra trains per hour into the existing peak service of 12 trains per hour through Cockburn. It might be better to run the Mandurah line peak service as
- every 15 minutes to Mandurah
- every 15 minutes to Rockingham (extending to Mandurah at end of peak)
- every 15 minutes to Cockburn
to avoid congestion in the turnback sidings and run everything south of the river on a 15 minute cycle
Special event services to the new stadium would take this route and Showgrounds special event services could be an extension of the Cockburn via Thornlie services west of Perth