At the beginning of the year, the Rail Engineer magazine (issue 136, February 2016) covered the Christmas and New Year programme of track delivery work, including a plain line possession on LNE that was proudly handed back at 125mph. At the time we mulled over the possibility that 125mph handbacks for switches and crossings (S&C) projects could be just a couple of years away.

Nine months later, and two years after the S&C North Alliance – the powerhouse partnership between AmeySersa and Network Rail – was formed, we’re celebrating the delivery of Britain’s first ever 125mph handback on an S&C project.

The expectation of higher handback speeds is becoming the norm, especially on busy passenger routes where the timetable is sensitive to train speed reductions. However, grasping those extra few mph (or km/h) takes a great deal of extra care. 25mph on top of 100mph may not seem much, but anyone involved in managing the dynamics of a train going at 125mph will appreciate just how fast things are happening.

On plain line, 125mph is relatively straightforward. High speed running over switches and crossings is another kettle of fish. To open an S&C renewal at 125mph takes careful planning. Many in the industry thought that this milestone would be years down the line.

The complications

Pway engineers can look away now while we go through the basic list of the fishes in the kettle. Pretty obviously there are the switches, which have to maintain their integrity and relative positions with the stock rails. There is a crossing which in itself could involve moving parts to avoid there being a gap to jump over. There is an assortment of bearer lengths, and the whole lot has to be tamped using a specialist S&C tamper. Oh and then there’s the signalling and OLE to contend with. So, S&C isn’t straightforward.

Nick Matthews is the programme engineering manager with the S&C North Alliance, and he explained what it takes. “You’re looking at the culmination of two years of work – we’ve taken a gradual approach to what is a major step change in S&C delivery. We’ve been quite cautious, by identifying the right test sites, techniques and applying the due diligence to succeed.”

Nick refers to “progressive assurance”. It means looking at each stage in a project and making sure that it is demonstrably checked against defined tolerances.

With each stage signed off as being within tolerance, the Authorised Person – the individual who has the responsibility at the end of the possession to decide on an opening speed – will have a complete history of each stage in the works.

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The issues that are critical to successful high-speed track handover at the end of a possession are largely hidden from view, buried in the ballast. Top and line may appear fine, but it is the quality of formation treatment and ballast compaction that will decide whether a train rides smoothly over newly laid track or whether it – or subsequent trains – experiences rough riding (or worse).

Compaction tracked

The S&C at Belford, on the East Coast main line (ECML) north of Newcastle, was chosen as the site to go live with progressive assurance. Between 16 and 19 September 2016, there were two worksites involved on the Down line at each end of the Down passenger loop – 506 points (facing) and, about a mile further north, 510B points (trailing). The latter set of points was planned for a 125mph handback. 506 points, being a facing connection and also close to Belford level crossing, was planned for 80mph.

Both sites were dug to 400mm and made good with a 100mm sand blanket followed by 300mm of bottom ballast compacted in one layer by a Variomatic Bomag roller. This particular machine gives an output trace of its performance and the stiffness of compaction achieved at every stage, which is retained as part of the assurance process.

At the ends of each excavation there is a transition length where the depth of dig tapers up to the existing bottom ballast level. A basic formula of line speed divided by seven gives the appropriate gradient. The purpose of the transition is to ensure that trains don’t ‘drop’ abruptly into a hole of relatively less compacted ballast and at the other end don’t suddenly hit a step. Although the possible differences in level are very small, trains travelling at high speed are likely to experience a rough ride.

The S&C modular units were then installed and aligned manually to within 5mm. Then followed the stone drop followed by a combined tamp/DTS (dynamic track stabilisation). The first pass is what is termed a disruption pass where every single bearer, including any hollow bearers, is tamped with the DTS following on in maximum mode to give maximum ballast consolidation. There’s then a second, very fine lining pass with the tamper, with the DTS in variable mode. Rather than giving maximum consolidation, it follows the geometry ramping up its action where slight track tolerance issues are found.

 

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DTS reputation

It may be worth reflecting for a moment on the DTS function. DTS machines (right) have been around for several decades. Indeed they formed the backbone of the high- speed handbacks of the 1980s. For our non-pway readers, DTS machines simulate the passage of trains by vibrating the track – a lot. But they had their detractors. They were perceived as being…brutal.

Whether this was a valid viewpoint could be up for debate. For track compaction they were doubtless very effective – because they were indeed brutal (allegedly). For lineside structures and buildings they were quite rough. They made the teacups rattle. For S&T equipment they were – or at least it was felt that they could be – downright destructive. For many, the perception persists to this day, and so it was necessary to be a little more scientific to measure what they are capable of doing – and undoing.

In our last article on this subject we referred to forthcoming trials to assess the true nature of the DTS beast. These were to be conducted through Network Rail’s central innovation team of IP Track with help from Southampton University – their ISVR Consulting.

Two sets of switches were chosen, located in Grange sidings near Stoke. The trial was run in February and June of this year. A 39-page report, with many tables and graphs, concluded that an appropriately used DTS should not cause any more vibrations than would be experienced in normal train running. However, the S&T equipment at Belford was taken off just in case…

And the first train is…?

When the time came for the Authorised Person to assess the track at Belford, he was in possession of a complete file of all the stages in the relaying process and their measured results. Progressive assurance proved its worth. Along with what could be seen and, just as importantly, a fact-based knowledge of what could not be seen, the temporary speed boards were ‘spated’ – that is, a cancelling indicator was shown.

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Despite the months of planning, it isn’t possible to anticipate what the first train to run over the relaying site will be. In the end, a freight train running at its speed of 60mph turned up. The site team didn’t have long to wait, though, for an empty coaching stock movement of an HST from Heaton Depot to Berwick, which duly rattled through with its throttle wide open for the clear line ahead. A very satisfying end to a well-planned endeavour.

Next up?

What’s next then? Whilst opening at line speed won’t be appropriate everywhere, making high speed handback BAU (business as usual) on S&C is realistically on the cards – it’s simply a matter of putting the pieces together – combining the people, techniques and technology to deliver more of the incremental improvements we’ve seen over the last few years.

The next big challenge will be to tackle the logistics of relaying a full crossover. For our non-pway readers, the complication here involves bearers that are long – very long. They are continuous under both tracks. Anything you do to one track affects the other whereas, with the single leads at Belford, none of the timbers extended under the adjacent track and the connection to the loop line was a modest 40mph. Nick acknowledges that opening a full crossover at 125mph will be challenging, but having tested the concept of progressive assurance and found it to be fit for purpose, there’s just the engineering to sort out.

And engineering’s easy, isn’t it?

Written by Grahame Taylor