With reliability on the Cityrail network sagging and extra capacity needed to handle the growing transport demand, the solution that the NSW government is seeking has been to reduce network flexibility (see the previous post on the 2013 timetable for an explanation of how reliability, capacity, and flexibility are interrelated).
Greater flexibility, the ability to get a direct train to the CBD from any station on the network, can reduce capacity and hurt reliability for a number of reasons. These include the need to have flexible (i.e. different) stopping patterns on the same track, the need to run separate lines on the same track, and the need to use flat junctions (see below). All of these things mean that increased flexibility also adds complexity to the network. Meanwhile, a simple system would require many passengers to change to a second or even third train in order to reach the CBD. This is a potentially unpopular option and is why Cityrail has opted for flexibility up until now.
Flexible stopping patterns
Some trains are express services, while others are all stations or limited stops services (which will collectively be called local services).
Often there are two pairs of tracks running side by side (such as from Central through to Hurstville or Parramatta), then one is generally designated for express trains and the other for all stops trains. This allows the express services to overtake the slower local services.
In other cases there is only a single track pair (such as much of the Northern Line between Hornsby and Strathfield), meaning that express services can get stuck behind local services. Here you must make a choice between flexibility (i.e. running both express and local services) or capacity (i.e. running just one type of service, generally local, at higher frequencies).
The Cityrail network operates 3 different “sectors”, effectively independent lines. However in some parts of the network (the section between Granville and Homebush in particular) it is necessary for trains from different sectors to mix in order to maintain flexible stopping patterns. This means that delays on one line can often spillover onto another line.
Take the huge delays that occurred 2 weeks ago that originated on the North Shore Line due to overhead wiring issues. This had immediate impact on the North Shore Line, and soon spread to the Northern and Western Lines whose trains feed into the North Shore Line. However, as Western Line trains also share track with the Inner West, South, and Bankstown Lines, these too soon began to suffer delays, which later flowed on further onto the Airport and East Hills Line. Only the Eastern Suburbs and Illawarra Lines, which is the only one of the 3 sectors to be completely separate, did not suffer delays.
Independent lines would not have prevented the delays, but it would have quarantined them and stopped them from flowing onto others.
Where two roads intersect at traffic lights the signals alternate so that only traffic on one road can move. For every minute that one road has a green signal, the other road loses a minute of green. This is effectively what happens in the world of railroads when a train crosses from one track to another that are separated by a third track in between them going in the opposite direction. These are referred to as “flat junctions” (see diagram below for an example), and every train that crosses one prevents another train from using the other track at that moment. You can avoid the use of flat junctions through the use of dives and flyovers (the best example of which can be found between Redfern and Central Stations), but these are not always available nor feasible to build.
The diagram above shows a section of rail near Macdonaldtown, right before the Western and Illawarra Lines merge. The maximum capacity of each track is 20 trains per hour, and each train that crosses a track via a flat junction effectively uses up one of those 20 hourly “slots” on the track that it crosses. So the 7 trains that move from the “Up Main” track to the “Up Suburban” track use up 7 slots on the “Down Main” track, restricting that third track to 13 trains per hour rather than the maximum of 20.
More importantly, conflicts due to flat junctions hinder reliability. When a train runs late across a flat junction, it delays not just the trains on its line, but also the line that uses the track that it crosses. The 7 trains that used the flat junction above can be timetabled in, but if one of them runs late and stops following the timetable then the delays flow on to the trains on the “Down Main” track, even though. These delays further constrain capacity, because for every 3 minutes of delay you effectively reduce the maximum capacity on a line by 1 train per hour. The line across the Harbour Bridge is a great example – here you often only see 17 trains crossing the Bridge during the busiest hour of the day even though 19 trains are timetabled, so actual capacity is 10% lower than timetabled.