Sunday, 12 December 2010

Backshunts and Headshunts (North Taieri Sidings Example)

Backshunts and headshunts are dead end pieces of track which are commonly connected to other shunting tracks and loops to facilitate train shunting operations. Whilst they superficially resemble each other and may be easily confused they are used differently and their names represents these differences.
The difference in naming relates primarily to whether the end of the headshunt or backshunt is entered by the head or back of the train, hence the names. I’ll use this map of part of the North Taieri rail sidings to illustrate this. Note that the map may be incorrect, however photos appear to confirm the main siding leg alongside Stedmans Road is single ended, with main line points at the south end only at the time of writing. In the working of such a siding, since the locomotive cannot be run around the train, the locomotive will have to be put onto the correct end of the train at Wingatui before the shunt goes up to North Taieri. The shunt will be propelled in one direction across two level crossings in the area.
N.B. The North Taieri sidings contain various loops so it would appear they are not actually worked as described here, and there may not be any propelling of shunts across three level crossings – Stedman Road, Factory Road and Gladstone Road North. However it is illustrative, and applies to many other siding layouts where single dead ended sidings are common.

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Looking at this map you can see the shunting leg alongside the main line starting from Silver Stream Bridge at the south end and going up to Dukes Road North at the north end. The main sidings being at the north and south boundaries of the industrial site to the left (Fonterra’s new stores).
The way the piece of dead end track at the north end of the shunting leg is used is as follows:
  • To service the south end sidings, the locomotive is put onto the north end of the rake and it heads north up from Wingatui. The locomotive heads the rake into the dead end track so it is functioning as a headshunt. The rake is then backed into the siding and wagons are added or removed. The headshunt is then used again as the locomotive heads out of the siding into the dead end. The train is then reversed onto the mainline and propelled back to Wingatui.
  • To service the north end sidings , the locomotive is put onto the south end of the rake and it is propelled up from Wingatui. If necessary the train at some stage is backed into the dead end track which functions as a backshunt. An example would be to uncouple part of the train or the locomotive while leaving the rest of the train in the backshunt. The locomotive or rest of the train then is put into the north end siding and when it comes out again the rear part of the train can be recoupled before the whole train heads south back to Wingatui.
A typical place you can see a headshunt used is in conjunction with a run around loop at a terminal station. The locomotive is taken off the train and goes forward into the headshunt, which is a short dead end section beyond the main line to loop points. The points are reversed and the locomotive then goes back into the loop and runs to the other end of the train on the main line. Whereas a backshunt is a proper description of a single ended siding into a site where the locomotive that works the siding is not kept in the site. The train is only ever backed into the dead end because the locomotive has to be put at the other end so that it can leave the site at the end of the shunt.
On a zig zag or switchback railway, dead end sections are found at the ends of the switches which function as either headshunts or backshunts depending on the direction of travel.
In former times, sidings such as North Taieri would have been worked in a more flexible fashion than is possible today by using fly or slip shunting, where wagons were allowed to be detached from a train while the train was in motion. This was standard practice at most railway shunting yards throughout the country. However the practice is quite dangerous as it requires rail operators to cross the tracks between moving wagons or to ride on the wagons themselves, and it has been phased out for a number of years, so that whenever the train is in motion, all wagons remain coupled and the train must be stopped completely before any wagons are attached or detached.
FOOTNOTES: North Taieri appears to be a bit more sophisticated than a lot of private sidings – it is apparently rated for mainline locomotives and must have one or more loops within the complex. It suggests that the site is expected to handle a high volume of rail traffic in the future.