Keep in mind that Sydney, which we seem to be modelling this on, doesn’t have line numbers for intercity services. They have three-letter codes, instead:
BMT Blue Mountains
CCN Central Coast & Newcastle
HUN Hunter
SCO South Coast
SHL Southern Highlands
Now, I’m not saying we should or shouldn’t do it this way, but it is a thing.
Wouldn’t T4 be Ipswich/Springfield and T3 Caboolture in this example?
I don’t think it really works, anyway. The Ferny Grove, Caboolture, Kippa Ring, Shorncliffe and Doomben lines can’t all be T3.
Regarding Ipswich/Springfield, would we give them the same number, like the T1 line in Sydney does for Penrith/Emu Plains and Richmond. Main line and branch share the same number.
From memory, it’s used during trackwork on the inner north when trains run from Caboolture to Doomben and back again.