1967 was the worst year for train accident fatalities in a decade. While derailments at Thirsk, Connington and Amble Junction contributed to the grim total of 82, most came from the calamity at Hither Green on 5 November when a piece of rail broke away just as a diesel electric multiple unit passed at 70 mph. Forty nine people were killed. Although the cause was the way that the track had been supported, it was realised more generally that the smaller wheels of diesel and electric locomotives and multiple units – combined with the high unsprung weight resulting from their axle-hung traction motors – had a more punishing effect on the track than steam locomotives: even allowing for the hammer blows of large unbalanced wheels.
Indeed, freight train derailments had become an unnerving part of the railway scene in 1960s Britain. There were 259 in 1966 alone, most of them involving traditional short wheelbase wagons on plain track. Arguably the causes of these derailments lay in the modernisation of both track and vehicles. Modern steel framed vehicles, while easier to maintain than their wooden counterparts, lacked their flexibility and were therefore more prone to a build up of oscillations – known as “hunting” – after encountering any imperfections in the track. Continuously Welded Rail (CWR), similarly easier to lay and maintain than the old sixty foot sections, brought even more rigidity. Its lack of joints also meant fewer natural breaks to disrupt oscillation build up. When these factors were combined with the sustained – and more immediate – high speeds possible with diesel traction and associated continuous braking, derailments increased alarmingly.
However, not all freight train derailments fitted the conditions described above. The 3 July 1967 incident at Thirsk – although involving steel framed cement wagons – was on jointed track while on 7 April 1964 a fully fitted train hauled by 9F 2-10-0 92161 had derailed on a plain line while coasting down the gradient between Appleby and Carlisle.
Research into hunting – and the general riding of railway vehicles – had been going on in earnest since July 1930. In that month the Advisory Committee on Scientific Research of the London Midland and Scottish Railway met for the first time and soon tasked Cambridge University with trying to solve the problem that would culminate in the tragedy at Thirsk.
That work had led to experiments with lower wheel conicities, which helped. But war intervened in 1939 and tests were curtailed. In 1962 however, Dr Sydney Jones joined the nationalised British Railways as Director of Research. Dr Jones – a radar expert and previously Technical Director of electrical engineers R B Pullin and Co Ltd – came to believe that hunting in wagons was similar to a dynamic instability suffered by aircraft known as “flutter”. He therefore set about recruiting a dynamicist from the aerospace industry – Alan Wickens.
Wickens and his team of aerospace engineers first looked at the ride of bogie vehicles but then studied the yaw and lateral damping of four wheeled wagons. Experiments with models then led to the building in 1966 of the four-wheel High Speed Freight Vehicle (HSFV1). This proved to be stable at up to 140 miles per hour when tested on the roller rig. HSFV1, Departmental running number RDB511023, regularly performed at 100 mph, both in tare and laden mode, whilst under test on the main line. Its suspension had two vertical coil springs and two vertical and one lateral hydraulic damper at each corner of the vehicle, together with a yaw control rod on each axlebox. It could carry various rail packs enabling it to be loaded to a variety of different states.
This concept was the fore-runner of the Class 142 railbus chassis, but more importantly results from various tests with HSFV1 allowed the wheel / rail interface to be better understood and this played a major role in the development of high speed operations leading to the design and building of the experimental Advanced Passenger Train (APT-E) and the ubiquitous InterCity 125 High Speed Train
Following the Thirsk accident, the suspension pioneered on HSFV1 appeared in an even more advanced guise on BR Ashford built Cemflo LA 236 which became known as HSFV3 and was now cleared to run at 75 mph. LA 236 was seen at Hoo Junction, Kent, in 1969 with the unusual livery of grey tank, silver solebar and maroon suspension components. It was HSFV4, a converted ferry van, however which truly proved the practicality of running long wheelbase two axle wagons at speeds of up to 75 mph. The first air braked wagons derived from this research work started to appear in 1969 and would go on to feature in British Rail’s “Air Braked Network” – rebranded as Speedlink in 1977.
By the 21st Century, traditional short wheelbase wagons had disappeared from what we now know as Network Rail but two freight train derailments on 15 October 2013 would shine the spotlight on small wheeled bogie container wagons
At about 02:40 on 15 October 2013, a freight train travelling from Birmingham to Felixstowe derailed close to the site of the former Primrose Hill station in north-west London. Fortunately, there were no injuries as a consequence of the accident, although there was damage to the train and to railway infrastructure. The North London route, which carries London Overground passenger services as well as freight trains, was subsequently closed for six days.
One wagon ( design code FEA) in the train ran derailed until the train reached a junction near Camden Road station. At this point, an empty container toppled off the wagon and damaged overhead line electrification equipment. The derailment was caused by a combination of the track geometry and condition, as well as the longitudinal and lateral asymmetric loading of the wagon which reduced its resistance to derailment on twisted track. The Rail Accident Investigation Board (RAIB) noted that the rules on loading FEA wagons had been relaxed following a derailment at Duddeston Junction in 2007.
Meanwhile, at about 20:15 on 15 October 2013, a container train operated by Direct Rail Services, derailed about 4 miles (6.4 km) south west of Gloucester station on the railway line from Newport via Lydney. It was travelling at 69 mph when the rear wheelset of the last wagon in the train derailed on track with regularly spaced dips in both rails, a phenomenon known as cyclic top. The train continued to Gloucester station where it was stopped by the signaller, who had become aware of a possible problem with the train through damage to the signalling system. By the time the train stopped, the rear wagon was severely damaged, the empty container it was carrying had fallen off, and there was damage to four miles of track, signalling cables, four level crossings and two bridges.
The immediate cause of the accident was a cyclic top track defect which caused a wagon that was susceptible to this type of track defect to derail. The dips in the track had formed due to water flowing underneath the track and although the local Network Rail track maintenance team had identified the cyclic top track defect, the repairs it carried out were ineffective. The severity of the dips required immediate action by Network Rail, including the imposition of a speed restriction for the trains passing over it, but no such restriction had been put in place. Speed restrictions had repeatedly been imposed since December 2011 but were removed each time repair work was completed; on each occasion, such work subsequently proved to be ineffective.
The type of wagon that derailed was found to be susceptible to wheel unloading when responding to these dips in the track, especially when loaded with the type of empty container it was carrying. This susceptibility was not identified when the wagon ( coded IDA) was tested or approved for use on Network Rail’s infrastructure.
In its 2014 report, RAIB commented that the rail industry needed “to better understand how container wagons and their payloads are interacting with the type of track faults that might sometimes be encountered.” As a result, Cross-Industry Working Group on Freight Derailments was set up including representatives from Network Rail, freight operators, , the Rail Safety and Standards Board (RSSB), Interfleet, Huddersfield University, Lloyds Register Rail and the Office of the Rail Regulator. In particular, this Working Group is examining the increases in bogie wagon usage over the past decade and changes to the standards covering their structural strength – the latter having resulted in vehicles which flex less and may be therefore less tolerant of twisted track. In the last 10 years there has been a 50% rise in container traffic, while the containers themselves have been getting taller ( from 8′ to 9′ 6″) and longer – with the original 20′ standard length now stretching to as much as 50′. These increases may also have a bearing on stability, although incidents in 2015 implicated the reliability of the retaining spigots used on container wagons. Research work is also ongoing into the effects of uneven container train loading.