Three different alignments have been proposed for the Northwest Rail Link (NWRL) over the years: via Strathfield, via Chatswood, and via Parramatta.
The first (via Strathfield) involved the line from Castle Hill linking up with the Northern Line around Cheltenham on the surface, allowing trains to travel into the CBD either via Strathfield or Chatswood. This was abandoned due to the requirement that the line be quadruplicating between Epping and Cheltenham in order to prevent that portion of dual track from becoming a bottleneck. Local opposition and a cost so high that tunnelling was a cheaper option led to this alignment being abandoned in favour of the second option.
The second (via Chatswood) is the currently planned alignment. It involves connecting the rail tunnels directly to the underground station at Epping, which means all NWRL trains must continue on to Chatswood and cannot divert to Strathfield. This reduces flexibility, but Cityrail’s Clearways program of sectorising the rail network into independent lines meant that flexibility wasn’t something Cityrail was looking for anyway.
The third (via Parramatta) was floated by Parramatta Council as a way of getting the Parramatta to Epping Rail Link (PERL) built. It involved building the NWRL through to Castle Hill, then sending it South to Parramatta, before going to Epping and continuing through to Chatswood and then St Leonards (avoiding the need for an expensive Second Harbour Crossing). Passengers heading into the CBD could change at Parramatta for express services. However, it also meant a longer trip for anyone heading to Macquarie Park or the North Shore.
All three options see capacity constraints for CBD trips: with the Western, Northern, and North Shore Lines all highly congested and near capacity.
Where do people from The Hills want to go?
The locations that Hills residents desire to travel to is ultimately what should determine which of the 3 options should be taken. For the purpose of determining this, work commutes will be taken into account (as data is most easily available for these, though the most recent data I was able to obtain was from 2001). I’ll be using Bus Contract Region 4 (see map below) as a proxy for The Hills, however this also includes areas further South such as Westmead, Northmead, Carlingford, etc. Calculations are included at the end.
Most Hills residents (57% ) work outside of large centres. The widespread nature of where their work is located means that public transport is unlikely to compete with the private vehicle for their work commutes. Nor should it, as these are the sorts of trips which require the flexibility of a car, rather than the capacity of public transport. The remaining 43% work in large centres [A], primarily in Parramatta/Westmead – 9.9% [A], the Global Economic Arc (Macquarie Park, Chatswood, St Leonards/Crows Nest, and North Sydney) – 7.7% [B], the Sydney CBD – 7.3% , Castle Hill – 4.7% [A], and various other centres – 12.6% . These are respectively shown in green, blue, yellow, grey, and brown in the chart below.
Note: The above diagram shows North Sydney as having a 42% jobs share. That is a typo. It should read 2.3%
Given the southern half of Region 4 includes suburbs between Parramatta and The Hills, which are likely to over represent the number of people who work in Parramatta/Westmead, the proportion of Hills residents who work in Parramatta/Westmead is likely to be less than 9.9%. That would make each of the 3 major employment zones (Parramatta/Westmead, the Global Economic Arc, and the Sydney CBD) are roughly equal in size, with Castle Hill close behind them.
How the different alignments stack up
All three options have the same alignment up to Castle Hill, at which point they begin to diverge. So it is the other 3 employment zones which differentiate the alignments.
The via Parramatta alignment is the only one that provides access to Parramatta/Westmead (the latter via a change of train at Parramatta). It also provides access to both the CBD (with a change of train at Parramatta) and the Global Economic Arc (by continuing on via the Parramatta to Epping Line). However, the former is capacity constrained and the latter would be delayed by having to travel to Parramatta before continuing to Epping.
The via Strathfield alignment gives no access to Parramatta/Westmead. By allowing some trains to go to the CBD via Strathfield and some via Chatswood, capacity constraints are limited. However, it also limits access to the Global Economic Arc. Eventually, construction of a Second Harbour Crossing can allow all trains to travel via Chatswood, providing good access to both the CBD and Global Economic Arc.
The via Chatswood alignment gives no access to Parramatta/Westmead. It gives the best access to the Global Economic Arc, initially with direct trains to Macquarie Park and Chatswood, but easily extended to St Leonards by quadruplicating the track between Chatswood and St Leonards. Eventually, construction of a Second Harbour Crossing can allow all trains to travel directly to the CBD, providing good access to both the CBD as well as the Global Economic Arc.
The via Parramatta option provides benefits if a Second Harbour Crossing does not happen, and is partly designed to defer the need for one. It also highlights why the government has committed to a Second Harbour Crossing – it unlocks much of the potential of the NWRL. This makes the via Parramatta option a viable one, but also one that suffers from short sighted vision, as a Second Harbour Crossing will eventually be needed, but will be less useful if there is no NWRL for it to connect to.
The via Strathfield and via Chatswood options seem roughly neck and neck, especially considering either can be upgraded with a Second Harbour Crossing to run trains directly to the CBD via Chatswood, providing good connections to both the Global Economic Arc and CBD. But there are 2 things that make the via Chatswood option superior. First, it avoids the problems of building the surface route between Epping and Cheltenham to avoid capacity constraints on that portion of track – including high cost of land acquisition, delays due to the need to start planning again from scratch on that portion of the line, and strong local opposition. Second, it goes against the concept of sectorisation, mixing different trains on the same lines – in particular this would prevent an effective private sector operation of the new line and the associated cost benefits that could come from it.
Each alignment has advantages and disadvantages, and there is no clear superior option. However, the NWRL via Chatswood alignment does appear to have a slight edge over the other options, on the assumption that a Second Harbour Crossing is built right after the NWRL is completed (as is current government policy).
However, this does not increase capacity on between the Hills to Parramatta, so improvements here should also be considered, particularly on the key Windsor Rd and Old Windsor Rd corridors. The former has a proposal for light rail linking Parramatta to Castle Hill currently undergoing a feasibility study, while the latter already has a T-Way where increased bus frequencies would easily achieve improved mobility.
: Contract Region 4 (page 15)
: Contract Region 7 (page 6)
[A]: “Of the workforce living in Region 4 approximately 43% work in major centres. Of those employed in centres, most were employed in…the centres of Parramatta (16%), Castle Hill (11%) and Westmead (7%)” 
Castle Hill: 11% x 43% = 4.7%
Parramatta: 16% x 43% = 6.9%
Westmead: 7% x 43% = 3.0%
Parramatta/Westmead: 6.9% + 3.0% = 9.9%
[B] 13.2% of Region 4 workers are employed in Region 7, which includes all 4 centres of the Global Economic Arc 
Region 7 employs 206,500 workers in total 
Each of the centres in the Global Economic Arc employ the following number of workers: Macquarie Park (26,814), Chatswood (19,842), St Leonards/Crows Nest (36,514), North Sydney (36,597) 
Macquarie Park: 13.2% x ( 26,814 / 206,500 ) = 1.7%
Chatswood: 13.2% x (19,842 / 206,500 ) = 1.3%
St Leonards/Crows Nest: 13.2% x (36,514 / 206,500 ) = 2.3%
North Sydney: 13.2% x (36,597 / 206,500 ) = 2.3%
Global Economic Arc = 1.7% + 1.3% + 2.3% + 2.3% = 7.7%