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I grew up in Billericay, Essex and I regularly caught the train from there into London.

At Shenfield, the line from London splits - the main tracks head north towards Ipswich, and the other tracks head out east towards Southend-on-Sea: Shenfield railway junction

One things has always puzzled me about this junction. There is a normal double track section on the branch line which operates in both directions, but there is also the single line flyunder which only operates in the eastbound direction - this flyunder is used far more frequently than the double line section in that direction.

Is anyone able to shed some light onto why this flyunder is there, and why it is used more than the other track?

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    My prime guess is as trains terminate at Shenfield, so they need a track(platform) to reverse. Likewise trains, from London, which don't stop at Shenfield, can get an uninterrupted way towards their destinations rather than waiting for the track to clear. – DumbCoder Aug 22 '16 at 8:10
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    I think this question is borderline off-topic on Travel and I'm not sure on what side of the the border. Either way, you might be interested in the Railways & Railroads proposal on area51 ☺. – gerrit Aug 22 '16 at 10:01
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    I'm voting to close this question as off-topic because it's about train routing. – JonathanReez Aug 22 '16 at 10:23
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    My guess would be, they use the flyunder so they don't have to cross the lines for traffic coming south from Ipswitch. Without the flyunder, either east bound Southend-On-Sea trains will have to wait at a red signal for southbound (from Ipswitch) trains or vice versa. Depending on how busy the line is this could cause quite a delay. So adding the flyunder increases the capacity of the line. – JenniP Aug 22 '16 at 10:38
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From page 105 of Network Rail's Sectional Appendix you can see a map of the station. From East to West there are five lines:

  • Slow line from London (DE on map)
  • Slow line to London (UE)
  • Mainline from London (DM)
  • Mainline to London (UM)
  • Southend to London line that joins the mainline south of Shenfield (USD)

Trains to London from Southend can come around the double track line that you identified and directly join the mainline to London, or join the London slow line by just crossing 1 line (Mainline from London).

In contrast, a train from London on the slow line (the majority of services) is on the "wrong" side of the station, and would have to cross three lines to get to the Southend Branch, so instead uses the flyunder. Trains using the mainline from London either cross one line onto the slow line away from London and use the flyunder, or cross the mainline to London and use the double track section towards Southend.

The flyover appears to be on 1930s OS maps so is rather older than I might have expected.

  • Thank you for your research! The NR map is very useful. Unfortunately the OS website doesn't appear to work properly. It's interesting to know how long the flyunder had been there for. – Virtual Anomaly Aug 22 '16 at 11:40
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    @lightswitchr The map page doesn't appear to allow linking to a particular view. If you search for Shenfield, you can load it and get two separate views. Click on the "left layer" button and choose a historic OS or 1930s OS, and you can also make the right panel a satellite image. Hove a play with the site, it's brilliant when you get the hang of it – Miff Aug 22 '16 at 12:31
  • I had HTTPS Everywhere enabled which broke the site. My bad. – Virtual Anomaly Aug 22 '16 at 13:11

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