Pulling over the Doomben line discussion from the Airtrain thread…
If we look at what’s done today we see that Doomben line trains cross at EJ (which is perfectly sensible). Then it’s something like 9 minutes to Doomben, 11-12 minutes’ dwell, 9-10 minutes to EJ, for a total cycle time of 30 minutes (the inbound train apparently leaves after the outbound).
We can plot this on a string diagram as below. (I’ve editorialised EJ a little so the cross happens at EJ rather than just outbound of it as the timetable implies; this might be incorrect but isn’t especially relevant to the key takeaway.)
Now let’s add an infill service (dashed line). This does assume nothing else is using EJ at that moment, but this should be doable.
The existing and infill trains cross between Ascot and Doomben!
Conveniently Ascot already has two platforms (perhaps P2 might need some upgrades). But we might not want to duplicate Ascot-Doomben with the two racecourse-entrance underpasses and the Nudgee Rd crossing.
Now, maybe it is possible to timetable the line differently, with a shorter dwell at Doomben. I understand QR’s shortest comfortable dwell is 8 minutes; let’s try that.
Now the cross is on the Hendra side of Ascot. The cross at EJ is also gone, moving between EJ and Wooloowin. This is probably OK as long as it doesn’t interfere with other services from Shorncliffe and Airport.
There’s swings and roundabouts with recovery time. An 8 minute dwell doesn’t have much to spare.
All in all, I think this argues for duplicating Ascot-Doomben to support 4tph (but it’s probably not necessary to duplicate Doomben itself).