This week we’re taking a look at the work our Test Car 2 was built for, namely the slip/brake testing of freight wagons.
For 50 years from the late 1960s the acceptance of new or modified wagons onto British Rail included dynamic brake testing predominantly carried out using the slip/brake method. This test measures the brake stopping distance performance of the wagon independently, i.e. not coupled with other vehicles in a train.
In order to conduct slip/brake testing a special coupling arrangement was used on Test Car 2, the design of which originated from use on North Eastern Railway J21 and J25 type steam locomotives providing banking assistance on the Darlington to Kirkby Stephen line over Stainmore Summit in the 1940s and 50s.
In addition to the slip coupling, the brake system on Test Car 2 was modified such that a test engineer could independently operate the brakes of the wagon under test without affecting the brakes of the test coach or locomotive which remained under the control of the driver.
A typical slip/brake test train would consist of a locomotive, Test Car 2 and the wagon under test. Class 47 locomotives were preferred for the slip/brake testing, not just because they had adequate power to accelerate quickly; also, the responsiveness of the engine control system of the Sulzer locos (compared to the English Electric locos) made it easier for the driver to control the speed more accurately with the short and often light trailing load of a test train.
The slip/brake tests were normally carried out on the Down Slow line of the West Coast Main Line between Crewe and Winsford. To ensure safe operation a ‘T3’ possession of the Down Slow line was applied by the Crewe Coal Yard signalman, making sure it was blocked to all traffic and protected by stop boards and detonators at each end of the possession section.
This section of line was particularly suited to the slip/brake testing because the test section was level, and the four-track formation allowed normal traffic to continue on the other three lines whilst testing was in progress.
It was not permitted to carry out slip/brake testing during hours of darkness, thick fog or falling snow and at least 1 mile clear visibility was needed before testing could commence; this was because the test engineer had to be able to clearly keep the test vehicle in sight at all times when the vehicle was detached from the test car.
A slip/brake acceptance tests of a wagon would normally be carried out over two days, one for each load condition, so one day for empty and one day for loaded wagon.
Each day would start with test engineers joining the test train at the Derby RTC, a loco would arrive from Derby 4-shed and would transit to Crewe. The train would pause in the station to pick up a Traction Inspector, (and some sandwiches) then continue onto the Down Slow line at Crewe Coal Yard. The T3 possession was then set-up to protect the line, the train brake systems were configured for test and static test checks for correct operation were carried out. These static checks included measurement of brake cylinder application timings and pressures on the wagon under test, and a continuity brake test on the locomotive and test car brakes.
Heavy duty instrument cables were attached from the test car, along the outside of the locomotive and into the leading cab (normally through the drivers’ cab-side window) and connected to twin intercom speaker/microphone outstations. The locomotive was always manned by a driver and a traction inspector for slip/brake testing; this allowed the test engineer to communicate using the intercom directly with the traction inspector in the cab so as not to distract the driver from his duties.
Once the intercom was tested and the static brake tests completed to the satisfaction of the test engineer, the slip coupling was fitted between the test car and the wagon, and testing could commence.
A slip/brake test is described in stages as shown in the diagrams below.
The test engineer calculated the wagon brake stopping distance for each slip/brake test run, taking the total distance from the point at which the brakes were applied on the wagon until the locomotive and test coach had stopped, then subtracting the distance the locomotive and test coach had to reverse back to collect the wagon. The results were plotted on a speed/distance graph.
The stopping distance limit in 1987 for a wagon running at 75 mile/h was 951metres, the graph below shows test results just failing to meet this, therefore this wagon would have been subject to modification to the brake system before a retest.
A deceleration meter fitted on the headstock of the wagon containing a calibrated accelerometer and a low pass filtered output onto a strip paper chart provided an accurate measure and characteristic of deceleration rate of the wagon throughout each slip/brake test.
Tests were always carried out at varying speeds from 30 mile/h up to the maximum operating speed of the vehicle under test. On average 15 slip/brake test runs could be completed during a day’s testing, however the achievable number of tests varied depending on how slick the team on board was working, the type of locomotive, the performance of the wagon under test, and more often than not, the weather.
After testing was completed the slip coupling was removed and a standard screw coupling re-attached, the brake system was configured back in the normal manner and the statutory brake continuity test completed before the train was hauled out of the test site. The train would then proceed up to Winsford for a loco run-round before returning via Crewe to the RTC at Derby.
In addition to wagons, on-track plant and coaching stock were occasionally slip/brake tested, for example this Cowans Sheldon 75t breakdown crane needed testing after it was modified by fitting air brakes in 1987.
Due to the fall in wagons requiring testing and UK network access constraints, slip/brake testing on the UK network finished by 2010. Computer modelling and dynamometers offer alternatives to testing now days although this type of testing does continue in mainland Europe and has also been done on private lines. An example were the tests done some years ago at the GCR, albeit without the need for a test coach since the computer took over. The link shows a Youtube video of a GCR slip/brake test run using D123 in July 2009. https://www.youtube.com/watch?v=s3UfKOhYSMk
Once the GCR is up and running again, keep a look out for events at Quorn when we hope to have Test Car 2 open to visitors again with displays and more information about its history and slip/brake testing.
For those with an appetite for more detail, the full story of Test Car 2’s work during the 1980s and 90s, along with accounts of why and how wagons were tested for acceptance onto British Rail, can be found in Dave Bower’s Rail Vehicle Testing book. ISBN 9781999935603 https://www.bowerbooks.co.uk/
Lastly, we await news from the GCR following the government announcement tomorrow whether we can re-commence working on the fleet again, continuing we’d left off on the Yellow Coach and 2 tank wagons. Either way We’ll be back with another feature, or hopefully an update. See you then.