A train of thought on several tracks.

                                        From Sockburn overbridge looking towards city centre

There is a strong push for the potential of commuter rail to be investigated more thoroughly in Christchurch. 

The various groups and individuals promoting it (NZ in Tranzit included) see it as a far more sensible option than the hugely expensive per kilometre on-street "light rail network" being mooted.

A major factor often mentioned is that there an existing current rail infrastructure to build upon.

I do not have any great deal of learning in rail systems, but enough enough background reading to know this would certainly not be as simple as it sounds.  Ideally peak hour services would need to be not less than every 20 minutes from Rolleston and Rangiora/Kaiapoi running to the city and to Ferrymead (near Heathcote). Adding up the commuter trains in both directions, during this peak period it appears to tie up a lot of rail line for about or six or seven hours a day, and would be be complicated at any time.

A major factor is West Coast coal trains and those heading for the Picton ferry will have their own timetable requirements, tied to passing loops and ferry sailings freeing up and filling up yards and locomotives. The coal trains will also have their own scheduling needs, closely tied I suspect to getting through multiple tunnels - including the Lyttelton tunnel and the Staircase tunnels and 8.5 km Otira tunnel through the Southern Alps. The latter tunnel needs banking with three extra locomotives to haul the hefty coal trains up the steep tunnel incline [NZ Railfan magazine had an exceptionally brilliant photo essay on this process a few issues back] . Just running this particular operation alone would take quite a lot of organised timing. Ideally, I presume, it works to a rythmnic pattern of arrivals and departures night and day.  Delays and interruptions of freight movements at Lyttelton can also cost thousands, with ships only in port for brief spells and containers etc needing to be stacked to patterns, if not for that ship for the next, or some other sequence. etc....in other words even though I don't know any specific details, all of these systems will be linked to timing, and maintaining workable margins of time to cover contingency.

There will be much more involved than just commuter train schedules. 

Sydney is spending multi-millions building a new South Sydney Freight corridor and is looking at upgrading the Northern Sydney line for the same reason - to free up trains delayed by commuter rail. Time is money! As small as Christchurch is, we have a tight corner in our Port which has to be accessed through central city lines.

I have proposed in past postings Christchurch should be pushing for an added line (or indeed double track corridor) between Redwood and Islington via the Airport.

Built before housing and along the airports noise zone boundary it is possible this could be gently trenched, with security fencing hidden from view and embankments either side (made from a tiny portion of the millions of tons of bricks and masonry rubble of our earthquake devastated city!). Presumably the same embankments could be covered in top soil and be planted in native bush and sound absorbing plants.  

This would offer reduction of immediate noise from both 24 hour a day freight trains and from late night flights landing/very early flights taking off for houses in the middle distance. Christchurch airport income partly relies upon arriving in the small hours budget flights, these being tail-ended on services to Melbourne or Australia from Europe or Asia, when aircraft would otherwise being sitting idle overnight at these major Australian airports.

In additional double-track line, Redwood to Islington, also gives Christchurch three active lines from the north. It is also possible judging from areas photographed for this page, that three or four through lines between Hornby could also be achieved , allowing parallel freight and commuter rail operations in most areas.

There certainly seems some capacity for extra lines from the photos above and below. Some of the sidings no longer connect to anything or have rubbish stacked on them. This suggests capacity for a third or fourth track or at least building lengthy passing loops capable of bearing full weight at reasonable speed.

The legit photo above is from the bridge itself - looking eastwards - shows a light weight siding/loop on the right, a disused siding/loop on the left.  Just discernible in the distance is the Alloy Street footbridge. This runs off a poorly marked and hard to find alleyway near Sockburn roundabout offering access to over top the rail pedestrian access to Parkhouse Road industrial area and Halswell.

Below the "illegal" view - definite no-no being on railway tracks nowadays, as I discovered  thanks to being apprehended and removed by two policemen one sunny Saturday morning back in 2010.Nonetheless they did not stomp on my camera or rip out the card, so I did get to keep the photo I wanted, showing that each of these side tracks has its own bay for passing under Sockburn overbridge

 Sockburn Overbridge from below - hard to believe this sliver of concrete carries some the heaviest traffic in the whole city!

The overbridge also has a one way road (on the Hei Hei bus route years ago).  Grade separated - potential for buses to run both ways under congestion to a small industrial area platform station also linked to the Parkhouse industrial area, and Wigram and Halswell side of the track

This is the city end of the Sockburn overbridge, which also has a one-way underpass for cars and trucks to access the industrial areas around Waterloo Road and Buchanans Road and Hei Hei residential area. This suggests potential to create a limited access (with traffic signals for buses to travel in the opposite direction) to a Sockburn station, with footbridge to the Parkhouse industrial enclave as well, or a Wigram side - drop off point for rail passengers the close proximity of all elements is obvious from the photo below.

With appropriate bus routes and an overhead walkways large areas could easily access the suggested rail loop covering much of the city from a simple station here 

I am skipping past the railway yards and through Addington here (not having photos on hand) to the former railway goods shed (all 10,000 square metres) between Durham St and Colombo Street overbridges. This building appears to have withstood the earthquakes and probably still has internal platforms.  I believe this would make a fantastic Christchurch Central Bus and Rail centre. Added reinforced concrete columns, mezzanine floors, long cathedral windows cut in the sunny side and refacing in the stone removed from all the collapsed churches, as well as many other saved materials and artifacts, and hanging gardens,  indoor  trees etc could make this a magificent memorial as well as hugely functional buildings, with electric shuttle buses every 5 minutes to the city centre driving right through the middle!! This could be the city exchange for through route bus transfers, albeit most routes will go through the central city, as well as a long distance coach centre, and railway station with a direct line to the airport and Addington City sports and events centre. The external view makes it has heaps of land for added through routes or external platforms, with ample room for freight to pass without needing to interact with passenger rail.

Further east - on the other side of the central city - the line passing through Waltham and across Ensors Road also seems to have room for additional lines or passing loops

In the event creating a terminus at Ferrymead was not seen as viable another option might be to terminate (and mid day park up) some services at Ensors Road, at a station beside the railway workshops opposite the Sullivan Avenue campus of CPIT. With added bus lanes and priority measures fast access to rail from Colombo Street via Tennyson and St Martins Road, and from eastern suburbs and Eastgate via Aldwins could be offered.

On the other hand creating a Park-and-Ride station at Ferrymead, and opposite the site I believe of the National Railway Museum has a beautiful synergy. This would link the approximate 20,000 people living on the Sumner Peninsula (so to speak) and around the Harbour with easy access to city, sports complex and airport as well as jobs elsewhere such as Birmingham Drive or Izone in Rolleston whilst bringing workers into the growing industrial zone around Radley, Woolston, Hillsborough and Ferrymead Bridge.

I am not a 100% sure but I think the National Railway Museum is to be sited at the spot marked X, opposite the current historic railway station and yards at Ferrymead Historic Park. The actual liners beyond here are part of Ferrymead's Heritage tram and rail network.

The City Council is investigating playing poker with a $400 million gamble on light rail.  I believe conventional commuter rail is a better hand all up.

Indeed three track through line access, north and south may represent three Aces, giving Christchurch a very grunty infrastructure, for industrial freight or for passenger movements.

Note again; this is purely speculation, floating some ideas. There may be factors in rail I misunderstand that render these suggestions meaningless....I feel more at home with bus issues