"What is a Timing Loop" I hear you ask. If you didn't ask that question, then "What are those T signs beside the track for" would have to be the other most popular question. The really short answer is that they are an important part of the train protection system on the Regional Fast Rail lines in Victoria...
You might have seen one of the signs below on your travels around the Victorian rail network. They signify the start of a timing loop.
A Timing Loop is a 100m long section of track, with a T sign marking the start. They work in conjunction with TPWS to protect trains from approaching crossovers too fast.
The timing loop is located one signal before the controlled signal that can indicate a 'medium speed' aspect, where the maximum line speed before the diverge is 160km/h. The signal at the junction is initially at stop. If the train passes though the timing loop at 100km/h or less, then the signal clears for the diverge. If it doesn't, it stays red, and the train gets tripped by TPWS.
The 100km/h value was chosen because it shows the train is under control, but isn't so slow to piss everyone (eg train drivers) off.
Timing loops are a section of track 100 metres long, and are located approximately 3 kilometres before a facing turnout on a running line where the maximum speed is greater than 130 km/h. Timing loops are used to prevent trains from exceeding the maximum permitted speed for a diverging route (in most cases, a turnout).
When a diverging route is set on a high speed running line, the junction signal does not 'clear' straight away - the signal will stay red. The signalling system needs to know that the train is under control of the driver before the route can be cleared and set. The signal control system determines this by forcing the driver to slow down to 100km/h over the length of the timing loop.
Failure to do so will result in the signal staying red. In a worst case scenario where the train does not reduce speed at all, the TPWS equipment will trip the brakes on the train, forcing the train to come to a stop and so preventing a derailment at the diverge due to an overspeed.
Once it is determined that the train is under control, the junction signal will clear.
The junction signal is referred to being 'approach operated' and 'speed proved' as the aspect displayed is dependent on the approach speed made by trains.
The 100km/h value for the speed test was decided upon as a compromise between 'normal speed' (no demonstration that the driver was in control of the train) and 'medium speed' (which would have increased travel times due to slowing down)
NOTE: If the signal at the diverge is set for the 'normal speed' route (eg straight through) then the train does not need to be speed proved. The train is instead allowed to carry though at 160km/h.
On the Geelong line there are 6 timing loops. Each of them is installed approximately 3 kilometres in advance of a facing crossover, where the maximum line speed for approaching trains is 160km/h. The crossovers are:
- Werribee down end: the timing loop on the West line for up trains
- Little River down end: the first timing loop is on the East line for down trains
- Little River down end: another timing loop on the West line for up trains
- Lara up end: the first timing loop is on the East line for down trains
- Lara up end: another timing loop on the West line for up trains
- Corio down end: one timing loop on the East line for down trains
You would notice that for the crossovers at Little River and Lara, that they are protected in both directions due to being located in the middle of high speed track. The crossovers at Werribee and Little River are only protected from one side - where trains approach at 160km/h. From the other direction, the maximum approach speed is 130km/h.
You may have also noticed that there are other crossovers between Werribee and Corio that are not equipped with timing loops. This is because trains only approach these crossovers at a maximum of 130km/h. The second set of crossovers at Little River up end and Lara down end are examples of this.
On other RFR lines, timing loops are also installed on the approaches to loops, for example Bungaree deviation on the Ballarat line, and the single to double line transitions on the Bendigo line.
Marcus Chadwick has published a paper TPWS – A Train Protection System for Regional Victoria that covers TPWS in much greater detail. The paper is available to view online at http://www.asiapacificrail.com.au/media/2006-irse-paper.pdf
The same author also gave a presentation on the same subject on 14 September 2005 to the Rail Technical Society Australasia / Institute of Railway Signalling Engineers. Notes from Mr Chadwick's presentation are also available online at http://www.rtsa.com.au/chapters/vic/meetings/rtsa-tpws-pres-pt2.pdf
These resources were used in the production of this article.