The suspension fitted to the majority of the four-wheel wagons of the period dated back to Victorian times and whilst it was acceptable for the rough and tumble of the steam railway and the slow speed trains which were the norm, with the advent of diesel and electric traction the limitations in speed of this primitive suspension was all to obvious. There were numerous derailments of these vehicles, in particular the Palvans were notoriously bad riding vehicles, but the mechanism of instability leading to derailment was not really understood.

With the completion of the RTC at Derby, various engineers were brought together and began to address these problems in a scientific way. The instability of the vehicles brought about by the higher running speeds was investigated and a one-fifth scale model of a four-wheeled vehicle with a novel suspension was built and tested on a similar-sized roller rig. The suspension allowed lateral and yaw flexibility between the wheelset and the frame for the first time and incorporated lateral damping.

HSFV1

Following these successful experiments, a full-sized version of the roller rig was constructed and in 1966 the four-wheel High Speed Freight Vehicle (HSFV1) was completed. This proved to be stable at up to 140 mile/h when tested on the roller rig.

HSFV1, Departmental running number RDB511023, regularly performed at 100 mile/h, both tare and laden, 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 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 build of the experimental Advanced Passenger Train (APT-E).

HSFV2 was not built but following the disastrous derailment of a Cemflo wagon at Thirsk on 31st July 1967 (in which accident the prototype diesel DP2 was written off) the suspension was improved and re-built in 1968 as HSFV3. It is not clear what happened to this vehicle.

HSFV4
HSFV4  was a former UIC long wheelbase 4-wheeled ferry van. Running number RDB 787319 it was also converted in 1968. It was equipped with a slightly different suspension  consisting of two coil springs with two 45 degree-inclined hydraulic dampers and a yaw control rod at each corner to allow some yaw freedom. The 35 tonne, 8m wheelbase wagon was similarly utilised for high speed trials at up to 90 mile/h.

High speed stable running was a great advantage in the days of wagonload traffic as the goods would still be on the pallets at the destination rather than spread all over the floor of the wagon, as they often were with friction link, long link suspension and other variants - even at 75mile/h maximum, since these type of suspensions could readily not control lateral movement nor hunting.

After the initial test periods HSFV4 was utilised for general test work and had a flat roof added with an access ladder from within the vehicle.  In this form it became a very useful van for carrying various equipment around the country in connection with OHLE and other testing work and was lettered 'Auxiliary Testing Vehicle (HSFV4)'. A bit of a comedown after its pioneering origins although it meant test trains could get to and from site at line speed.

Although they were excellent riding vehicles in the railway of the day they were viewed as experimental only and far too expensive for widespread adoption. It was also thought that the sophisticated suspension would not stand up to the daily rigours to which freight vehicles were subjected.

HSFV5

However in the late 1970 these experiments resulted the high speed freight vehicle  'production'  design. It was fitted to ten covered air braked vans (COV-AB) built at Shildon and was known as HSFV5. It consisted of a long double leaf  'Taperlite' spring suspension with hydraulic dampers inclined at 45 degrees to control both vertical and lateral movement *. It also replaced the nested springs with rubber auxiliaries. On test these vehicles were capable of stable running at up to 90 mile/h but in service were limited to 75 mile/h due to them running with other UIC long link suspension vans.