This topic concerns the pressure pulse produced by one train on another being passed on an adjacent line.
Although studies of this phenomenon had been undertaken for research and development purposes during the 1970s, a need to quantify the magnitude of the effect for existing and future high speed service routes arose in the late 1980s due to adverse comments from train users. The comments were relatively rare, but mainly centred around passengers being startled by the banging of doors (particularly of external sliding doors used on some types of Multiple Unit) and windows (particularly hopper windows) when passed by other trains at high speeds. In addition, coffee and other drinks resting on tables on the side adjacent to the Fast line, mainly in other HSTs, were regularly spilt by passing HSTs. This was caused by a rapid displacement of the coach wall against which the tables rested.
Although the events could not be called serious, it was evident that a criterion was needed for the design of new trains for the:
i) Door and window mounts and for the structural side-wall stiffness of vehicles likely to be operating on high speed routes
ii) Future high speed train nose shapes, (as it was known that it was the aerodynamic shaping, as well as speed, of the source train that sized the pulse magnitude).
Subsequently, tests were undertaken by the Research Division of BRB in 1988 to assess the magnitude of the largest pressure pulses produced by service trains at that time. Tests were undertaken on ECML with a test vehicle being passed, during both static and moving tests, by a number of service trains. Of particular interest was HST, as it was often the offending train and was operating at speeds up to 125 mi/h on tracks at a nominal spacing of 3.4m. In some places, track spacing was known to be less than this and, of course, considerably more than this in other places.
In addition, the Class 91 loco was being produced and it was necessary to choose a criterion bearing in mind future operation of the IC225 train (also on ECML). In that event, it was decided during discussions between the senior managements of the Research Division and the IC225 Project Team that IC225 operation at 225 km/h should form the limiting condition for defining the pulse limit. At that time, prior to tests being undertaken with Class 91, it had been assumed that the pulse characteristics generated by the nose shape of the Class 91 would be similar to HST, and therefore that a criterion based on an HST result scaled up from 125 mi/h to 225 km/h (140mi/h) should be adopted.
Results from the tests produced a mean value, (taken over several passes at different track spacings and speeds of both trains), for the HST normalised to 3.4 m nominal track interval, which was given by the non-dimensional parameter, ï„CP = 0.6. At 225 km/h, this equated to 1.44 kPa peak-to-peak amplitude.
Subsequent tests with IC225 showed the Class 91 to have slightly better characteristics than HST, but the 1.44 kPa value was adopted for future project design purposes. An indication of this is given in the attached letter involving a proposed lC250 development for WCML operation written by the Technical Director (Research) of British Rail Research to the Project Director IC225.
It is important to note that, in this letter and elsewhere, the 1.44 kPa criterion was defined in association with 3.4 m track spacing. Similarly, acceptance tests undertaken during development work on new train designs were checked against a limit of 1.44 kPa at 3.4 m track spacing.
Further, BR Research advised that, for practical purposes during track tests, compliance with the criterion was to be checked against a measurement taken at mid-window height on a stationary observing train on straight track on a calm (no wind) day. The result then was to be corrected to nominal 3.4m track spacing.
In the same way as for the original tests and for the nominal service condition chosen by Research and DM&EE management, there will be circumstances now when 1.44 kPa is exceeded. For example, movement of the observing train, the presence of cross-winds, reduced track spacing and track curvature can all increase the pulse amplitude. Thus, it is important to adopt this specification of the reference set of conditions under which the criterion is to be met.
Note that the above implies that rolling stock operating on high speed routes should be structurally designed to a criterion in excess of l.44kPa for the train passing pressure pulse case. For the proof load case of unsealed trains, this will usually be covered by the Q.5kPa specification for vehicle body structures (see Railtrack Gp. Stds. GM/TT0l22, GM/TTOl23, GM/RC2504). Sealed trains will be covered by their own more stringent limits. However, fatigue load cases particularly for unsealed trains may need to incorporate higher values associated with regular exceedances of the 1.44 kPa value.
It would appear, therefore, that the original Railtrack Spec. for WCML mistakenly omitted reference to 3.4 m track spacing in its definition of the conditions under which the 1.44 kPa criterionÂ should be met. Incidentally, the corresponding Railtrack Spec. for ECML does define 3.4 m as the reference condition.