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Tandem tamping

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After the success at Belford last year, where newly relaid switches and crossings (S&C) were handed back at 125 mph line speed using 928001, a Plasser & Theurer Unimat 09-4×4/4S Dynamic Tamper, Swietelsky/Babcock Rail (SBR) has continued its innovative collaboration with the S&C North Alliance to deliver another 125mph handback of an S&C renewal, this one at Sandy on the East Coast main line.

This next stage involved the use of two 09-4×4/4S Dynamic Tampers in tandem in order to tamp two crossovers, one between Up Fast and Up Slow during week 45 and the other between Down Fast and Down Slow in week 46.

Due to the long bearers in S&C, there is a need for tandem lifting to achieve optimal track quality. In this procedure, as the terminology suggests, both S&C units are lifted at once, using one tamper on each line. In this way, the whole length of any through bearer is lifted to the correct degree at the same time, obviating the risk of distorting the bearers or leaving them unevenly supported after the tamping is done.

Tandem lifting is now a tried and tested procedure in the UK. However, using a high output satellite machine with Dynamic Track Stabilisation in tandem is believed to be a world first.

Why parallel tamp?

Parallel tamping ensures an even force is applied across S&C during the lifting cycle. Single machine tamping on crossovers can lead to excessive load on the clips, causing them to deform as the bearers are not lifted properly.

Single line tamping can also leave voids under bearers, where ballast consolidation is most needed. And, of course, using two tampers means that switch and crossing layouts can be tamped more rapidly as well as to a higher quality.

928001’s sister machine, 74002, was introduced to the UK infrastructure in Dec 2017 and arrangements were made with LNE planners to stable the machines together at Welwyn Garden City for training and commissioning ready for the tandem lift/DTS in week 44.

Work carried out on the machines at Welwyn Garden City included carrying out modifications including hard wiring, and the fitment of the Tandem Lead to connect the machines during the tandem lift. Additional cameras were installed as were program updates to Plasser & Theurer’s PIC control system. This state- of-the-art microcontroller system gives the Plasser engineers remote access to rewrite the programmable logic controller (PLC) of each tamper, making the machines future-proof for further upgrades and reducing the need for additional hard wiring.

As part of the upgrade, an additional section of the working-cab screen for tandem tamping was developed by SBR and Plasser.

Experience from previous tandem works, using 08-4×4 machines in the fleet, had shown that the process can be slowed down significantly due to miscommunication between the crewmembers involved. SBR and Plasser have therefore developed an indication system which now gives the master machine operator a visual ‘confirmation’ when all operators are ready to complete or finish the work cycle, reducing the verbal communication needed.

This, coupled with the high output of the machines, now makes for a much more efficient process in comparison with conventional S&C machines in the UK. Simulation runs were carried out and a method was developed between the trainers and operators.

Some further testing was carried out for a future upgrade for tandem drive, reducing site entry and exit times.


Over a period of two weekends, the machines were deployed to assist S&C North Alliance with a major upgrade of a set of switch and crossings at Sandy.

The crossover rails were lifted synchronously at the through bearers by the two machines acting in tandem, communicating via the interconnecting cable. Long bearers that didn’t run through both tracks were managed using the third-point lifting arms of the machines, which lift the rails so that the fastenings are not subjected to excessive stresses and the bearers are correctly aligned.

Why Tandem DTS?

During conventional S&C renewals, the track is handed back at a reduced speed (TSR or Temporary Speed Restriction) due to a reduction in the lateral stability of the track. That is caused by the disturbance that lifting and tamping inevitably inflicts upon the ballast. Track quality and stability relies on the track structure and ballast and, traditionally, a TSR was kept in place until it was judged that enough traffic had passed to re-consolidate the ballast by means of the vibration caused by the passage of trains.

Dynamic Track Stabilisation (DTS) was developed to provide the means to re-consolidate the ballast artificially, immediately after tamping or other ballast disturbance. It can simulate over 140,000 tonnes of passing traffic, rapidly creating a greater uniformity of stone consolidation. By doing this, it is possible to reopen track safely to traffic immediately afterwards with a much higher TSR, or even with no TSR at all.

Originally, separate DTS machines were used that followed after a tamper, consolidating the track that had been tamped by one or more passes through the site to deliver the desired effect. The machines had trolleys that were lowered onto the two rails. They had roller clamps that allowed the trolleys to roll along the track whilst applying both horizontal and vertical loads to the rails. The machine applied both a vertical pre-load to the track and, at the same time, a vibratory horizontal and vertical shaking effect. This last was achieved by the use of rapidly rotating eccentric weights attached to the trolleys. The result was rapid consolidation of the ballast as already described.

The 09-4×4/4S tamper is itself fitted with DTS units which operate in a similar manner to those of the conventional separate machines. They allow the tamper to stabilise the track as it progresses, saving time by comparison with the use of a separate machine. In a further refinement, unique to Plasser machines, these tampers can apply variable DTS parameters ‘intelligently’, according to the needs of each increment of the track as they progress.

This variation in the DTS effect is achieved by variable imbalance of the rotating flywheel that causes variation in the intensity of the shaking imparted to the track. The downward pressure of the unit on the track to locate the lifted sleepers into the ballast bed is also varied appropriately in synchrony.

The combined effect causes flow of the ballast into a more stable position than was left after the actual tamping, whilst causing minimal change to the corrected track geometry left by the machine’s tamping bank.

The latest intelligent DTS units, as fitted to SBR’s 09-4×4, can now be stopped and started or varied in intensity during S&C treatment. Therefore, the machine doesn’t need to carry out additional passes through the site, saving valuable time during tight possessions. The variable DTS technique is applied intelligently, for example to remove minor high spots on a rail by applying higher intensity directly on the high points and reduced intensity in between them. It can also be used to avoid re-treating a rail during tandem working when, for example, the machines will sometimes travel over the same rail more than once whilst tamping the whole of a crossover.

The use of DTS technology is mandatory on high speed track in Europe and ensures the removal of ballast bed inconsistencies, increasing linear and longitudinal integrity after tamping for a longer maintenance interval between tamps, usually extended by around 30 per cent. Combined with the possibility of re-opening the railway at full speed immediately after renewal, this must be a winning formula!

Written by Chris Parker and Chris Tucker, a trainer and assessor at SB Rail.


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