There has been much criticism of the ride comfort of the most recent trains brought into use on the British rail network such as the class 8XX Inter City Express Trains (IET). Critics compare them unfavourably with older trains which, they say, deliver a superior ride. When travelling around the UK in 2022, your writer particularly noticed that IETs sometimes rode well and sometimes were extremely rough, so this is probably not entirely a train issue. Critics have also complained about uncomfortable seats (Issue 176 – July 2019), and these are clearly part of the perceived problem.
Passengers’ perception of ride comfort arises from a combination of their reaction to the accelerations they feel in three dimensions, and other issues such as noise and temperature. Ride comfort may be more subjective than objective, but there continues to be research to understand the contribution of the various factors to a ‘comfortable ride’ – vertical, lateral, longitudinal accelerations and jerks, together with noise. Illustrating this with one scenario, which does the passenger perceive as worse – general disturbance of the vehicle traversing points at speed or the same with much clattering from somewhere underneath the vehicle?
This article is in three sections. First, a case study of a 30-year-old fleet that was criticised for a poor ride when new but was improved; second, exploring some of the factors that might explain why we achieve a less good ride on modern trains compared with, for example, those in the case study; and third, what can be done to improve the new trains.
Case study: Mark 4 coaches
Mark 4 coaches were built by Metro-Cammell in the late 1980s and featured SIG type BT41A bogies. In service, the fleet was criticised for poor ride. British Rail management of the day accepted the criticism and determined that something needed to be done. As far as Rail Engineer is aware, the specification/requirements had been complied with, but it was clear from contemporary commentators (Roger Ford et al) that the ride was not good at 125mph, although there were reports that all was fine at the specified 140mph.
Investigation involved measuring accelerations using tri-axial accelerometers and recording equipment (such as is used for measuring the ride comfort of IMechE Railway Challenge locomotives) at various locations along the vehicle floors. This equipment is simple and straightforward to use. The analysis of the data indicated two problems.
First, there was a great deal of relative lateral movement between coaches especially noticeable if negotiating the inter-car gangway. This was the first time that the two halves of the gangway were physically connected and there were no friction-mating faces that would have provided a level of friction damping. The problem was resolved by fitting a transverse hydraulic damper underneath the gangway with one end connected to one coach and the other end to the next coach. The damper also improved the overall lateral ride.
Second, it was found that the yaw dampers mounted on both sides of the bogies were exciting the first vertical body bending mode. This is illustrated in the diagram showing the original orientation of the dampers as a red line and the body bending represented by the yellow dotted line.
Engineers investigating this believed that if the damper mounting location could be changed slightly, the problem would be reduced or rendered ‘mostly harmless’. The agreed modification was for the bogies to be turned though 180 degrees, meaning that the yaw dampers faced toward the coach centre rather than the end (the green line on the illustration). With the support of the supplier who had the models, the solution was ‘tested’ using vehicle dynamics multi-body computer modelling and shown to be effective. Simulations are often part of the solution, and they can really only be done by the supplier, the organisation that understands how the suspension works, the detail of which is key to effective simulation.
It was necessary to reposition the yaw damper body mounting bracket from the outer to the inner side of the body bolster, a comparatively straightforward modification. This is the configuration that will be seen on the remaining mk 4 coaches in use today, although observers will note that the shorter Driving Van Trailers retain the original yaw damper layout as they react to the yaw damper differently from the longer passenger cars.
Why do we achieve a less good ride today?
There are three main reasons: compliance culture, organisational complexity, and lack of a guiding mind.