Editing Great Western Main Line (section)

==Infrastructure==
[[File:St James Railway Bridge, Bath.jpg|thumb|right|St James Railway Bridge, Bath]]
There are four tracks between London and Didcot, two in each direction. The main lines are mostly used by the faster trains and are on the south side of the route. The relief lines on the north side are used for slower services and those that call at all stations, as only London Paddington, Slough, Maidenhead, Twyford, Reading and Didcot Parkway stations have platforms on the main lines (although a few others have main line platforms that can be used in an emergency). Between Didcot and [[Royal Wootton Bassett]], a series of passing loops allow fast trains to overtake slower ones. This section is signalled for bi-directional running on each line but this facility is usually only used during engineering working or when there is significant disruption to traffic in one direction.<ref name=2007plans/>

The summit of the line is at Swindon and falls away in each direction: Swindon is {{convert|270|ft|m}} above Paddington and {{convert|292|ft|m}} above Bristol Temple Meads. The maximum [[grade (slope)|gradient]] between Paddington and Didcot is 1 in 1320 (0.75{{nbsp}}[[‰]] or 0.075[[Percent sign|%]]); between Didcot and Swindon it is 1 in 660 (1.5{{nbsp}}‰ or 0.15%), but west of Swindon, gradients as steep as 1 in 100 (10{{nbsp}}‰ or 1%) are found in places, such as [[Box Tunnel]] and to the east of {{stnlnk|Dauntsey}}.{{sfn|MacDermot|1927|pp=124, 127}}{{sfn|Gradient Profiles|2003|loc=figs. W1, W6}}

The line is electrified between Paddington and Langley Burrell (just east of Chippenham) using {{nowrap|[[25 kV AC]]}} overhead supply lines; the Reading to Taunton line (as far as Newbury) and the South Wales Main Line (as far as Cardiff Central) are also electrified.

The line speed is {{cvt|125|mph|km/h}}.<ref>{{cite web |title = About Great Western Main Line |url = http://www.agilitytrains.com/great-western-main-line |publisher = Agility Trains |access-date = 20 December 2015 |archive-url = https://web.archive.org/web/20151222115910/http://www.agilitytrains.com/great-western-main-line |archive-date = 22 December 2015 |url-status = dead |language = en-GB}}</ref> The relief lines from Paddington to Didcot are limited to {{cvt|90|mph|km/h}} as far as Reading, and then {{cvt|100|mph|km/h}} to Didcot. Lower restrictions apply at various locations.<ref name=2007plans>{{cite web |title = Route Plans 2007 Route 13 Great Western Main Line |url = http://www.networkrail.co.uk/browse_documents/BusinessPlan2007/PDF/Route_2013_Great_Western_Main_Line.pdf |publisher = Network Rail |access-date = 20 December 2015 |archive-url = https://web.archive.org/web/20120925215820/http://www.networkrail.co.uk/browse_documents/BusinessPlan2007/PDF/Route_2013_Great_Western_Main_Line.pdf |archive-date = 25 September 2012 |url-status = dead |language = en-GB}}</ref> The line is one of two [[Network Rail]]-owned lines equipped with the [[Automatic Train Protection]] (ATP) system, the other being the [[Chiltern Main Line]].<ref>{{cite web |title = Great Western Main Line ATP Pilot Scheme |url = http://www.traintesting.com/GWML%20ATP.htm |publisher = Train Testing |access-date = 20 December 2015 |archive-date = 23 December 2015 |archive-url = https://web.archive.org/web/20151223114737/http://www.traintesting.com/GWML%20ATP.htm |url-status = live}}</ref>

=== Tunnels, viaducts and major bridges ===
Major civil engineering structures on the Great Western Main Line include the following.<ref name=":0">{{Cite book |title = Railway Track Diagrams Book 3 Western |last = Bridge |first = Mike |publisher = Tackmaps |year = 2010 |isbn = 978-0-9549866-6-7 |location = Bradford on Avon, UK |language = en-GB |pages = 1–5}}</ref>
{| class="wikitable"
|+Tunnels, viaducts and major bridges on the Great Western Main Line
!Railway structure
!Length
!Distance from London Paddington
!Location
|-
|Subway Tunnel (LU)
|{{convert|117|yd|m}}
|{{convert|0|mi|67|chain|km}} – {{convert|0|mi|73|chain|km}}
|West of Royal Oak
|-
|Spring Bridge Road Car Park Tunnel
|{{convert|121|yd|m}}
|{{convert|5|mi|70|chain|km}} – {{convert|5|mi|76|chain|km}}
|West of [[Ealing Broadway station|Ealing Broadway]]
|-
|Hanwell Viaduct
|{{convert|44|yd|m}}
|{{convert|7|mi|35|chain|km}} –  {{convert|7|mi|38|chain|km}}
| rowspan="3" |West of [[Hanwell railway station|Hanwell]]
|-
|[[Wharncliffe Viaduct]]
|{{convert|297|yd|m}}
|{{convert|7|mi|43|chain|km}} – {{convert|7|mi|56|chain|km}}
|-
|Hanwell Bridge
|{{convert|4|chain|m}}
|{{convert|8|mi|00|chain|km}} – {{convert|8|mi|04|chain|km}}
|-
|[[Maidenhead Railway Bridge|Maidenhead Viaduct]] ([[River Thames]])
|{{convert|237|yd|m}}
|{{convert|23|mi|21|chain|km}} – {{convert|23|mi|32|chain|km}}
|East of [[Maidenhead railway station|Maidenhead]]
|-
|Seven Arch Viaduct
|{{convert|68|yd|m}}
|{{convert|31|mi|19|chain|km}} – {{convert|31|mi|22|chain|km}}
| rowspan="2" |West of [[Twyford railway station|Twyford]]
|-
|River Loddon Viaduct
|{{convert|70|yd|m}}
|{{convert|31|mi|43|chain|km}} – {{convert|31|mi|46|chain|km}}
|-
|Kennet Bridge ([[Kennet and Avon Canal|Kennet & Avon Canal]])
|{{convert|4|chain|m}}
|{{convert|34|mi|77|chain|km}} – {{convert|35|mi|01|chain|km}}
|East of [[Reading railway station|Reading]]
|-
|[[Gatehampton Railway Bridge|Gatehampton Viaduct]] (River Thames)
|{{convert|99|yd|m}}
|{{convert|44|mi|00|chain|km}} – {{convert|44|mi|05|chain|km}}
|East of [[Goring & Streatley railway station|Goring & Streatley]]
|-
|[[Moulsford Railway Bridge|Moulsford Viaduct]] (River Thames)
|{{convert|147|yd|m}}
|{{convert|47|mi|27|chain|km}} – {{convert|47|mi|34|chain|km}}
|East of [[Cholsey railway station|Cholsey]]
|-
|River Avon Viaduct
|{{convert|72|yd|m}}
|{{convert|90|mi|77|chain|km}} – {{convert|91|mi|00|chain|km}}
|East of [[Chippenham railway station|Chippenham]]
|-
|[[Chippenham viaduct]]
|{{convert|90|yd|m}}
|{{convert|94|mi|08|chain|km}} – {{convert|94|mi|13|chain|km}}
|West of Chippenham
|-
|[[Box Tunnel]]
|{{convert|1|mi|1452|yd|km}}
|{{convert|99|mi|12|chain|km}} – {{convert|100|mi|78|chain|km}}
| rowspan="2" |Between Chippenham and [[Bath Spa railway station|Bath Spa]]
|-
|Middle Hill Tunnel
|{{convert|198|yd|m}}
|{{convert|101|mi|39|chain|km}} – {{convert|101|mi|48|chain|km}}
|-
|Sydney Gardens East Tunnel
|{{convert|77|yd|m}}
|{{convert|106|mi|24|chain|km}} – {{convert|106|mi|28|chain|km}}
| rowspan="3" |East of Bath Spa
|-
|Sydney Gardens West Tunnel
|{{convert|99|yd|m}}
|{{convert|106|mi|29|chain|km}} – {{convert|106|mi|33|chain|km}}
|-
|Dolemeads Viaduct
|{{convert|355|yd|m}}
|{{convert|106|mi|49|chain|km}} – {{convert|106|mi|60|chain|km}}
|-
|Arches and St James Viaduct
|{{convert|600|yd|m}}
|{{convert|106|mi|68|chain|km}} – {{convert|107|mi|20|chain|km}}
|West of Bath Spa
|-
|Twerton Viaduct
|{{convert|638|yd|m}}
|{{convert|108|mi|29|chain|km}} – {{convert|108|mi|58|chain|km}}
| rowspan="4" |Between [[Oldfield Park railway station|Oldfield Park]] and [[Keynsham railway station|Keynsham]]
|-
|Twerton Short Tunnel
|{{convert|45|yd|m}}
|{{convert|108|mi|70|chain|km}} – {{convert|108|mi|72|chain|km}}
|-
|Twerton Long Tunnel
|{{convert|264|yd|m}}
|{{convert|109|mi|03|chain|km}} – {{convert|109|mi|15|chain|km}}
|-
|Saltford Tunnel
|{{convert|176|yd|m}}
|{{convert|111|mi|57|chain|km}} – {{convert|111|mi|65|chain|km}}
|-
|St Annes Park Arches Viaduct
|{{convert|4|chain|m}}
|{{convert|115|mi|25|chain|km}} – {{convert|115|mi|29|chain|km}}
| rowspan="7" |Between Keynsham

and [[Bristol Temple Meads railway station|Bristol Temple Meads]]
|-
|St Annes Park No.3 Tunnel (or Foxes Wood Tunnel)
|{{convert|1017|yd|m}}
|{{convert|115|mi|58|chain|km}} – {{convert|116|mi|25|chain|km}}
|-
|St Annes Park or (Bristol) No.2 Tunnel
|{{convert|154|yd|m}}
|{{convert|116|mi|41|chain|km}} – {{convert|116|mi|48|chain|km}}
|-
|Main River Viaduct (River Avon)
|{{convert|108|yd|m}}
|c. {{convert|117|mi|24|chain|km}}
|-
|Main Down Viaduct (River Avon)
|{{convert|141|yd|m}}
|{{convert|117|mi|21|chain|km}} – {{convert|117|mi|27|chain|km}}
|-
|The Feeder
|
|{{convert|117|mi|51|chain|km}}
|-
|[[Bristol Harbour|Floating Harbour]]
|{{convert|3|chain|m}}
|{{convert|118|mi|16|chain|km}} – {{convert|118|mi|19|chain|km}}
|}

==== Line-side monitoring equipment ====
Line-side train monitoring equipment includes hot axle box detectors (HABD) and ''Wheelchex'' wheel impact load detectors (WILD), sited as follows.<ref name=":0" /><ref>{{Cite web |url = http://www.railwaycodes.org.uk/misc/habdwild.shtm |title = HABD and WILD equipment |website = railwaycodes.org.uk}}</ref>
{| class="wikitable"
|+Line-side monitoring equipment on the Great Western Main Line
!Name & Type
!Line
!Location (distance from Paddington)
|-
| rowspan="2" |Maidenhead HABD
|Up Relief
|{{convert|24|mi|03|chain|km}}
|-
|Up Main
|{{convert|24|mi|10|chain|km}}
|-
|Waltham WILD
|Up Relief, Down Relief, Up Main, Down Main
|{{convert|26|mi|21|chain|km}}
|-
|Twyford HABD
|Down Relief, Down Main
|{{convert|32|mi|02|chain|km}}
|-
|Basildon HABD
|Up Relief, Down Relief, Up Main
(Down Main disconnected December 2016)
|{{convert|43|mi|42|chain|km}}
|-
|Cholsey WILD
|Up Relief, Down Relief, Up Main, Down Main
|{{convert|49|mi|05|chain|km}}
|-
|Wantage Road HABD
|Up Main
|{{convert|59|mi|57|chain|km}}
|-
|Bourton HABD
|Down Main
|{{convert|72|mi|20|chain|km}}
|-
|Studley HABD
|Up Main
|{{convert|81|mi|40|chain|km}}
|-
|Twerton HABD 
|Down Main
|{{convert|108|mi|60|chain|km}}
|}