PRESSURE COMFORT CRITERIA FOR RAIL TUNNEL OPERATIONS .sx R G GAWTHORPE .sx Head of Aerodynamics , British Rail Research .sx Derby , UK .sx ABSTRACT The discomfort felt on the ears by passengers when travelling through tunnels has now reached a stage where limits of pressure change have to be applied .sx The choice of limit is not straightforward because of the variation in perception of discomfort felt between one person and another and also on different journeys depending on the journey characteristics .sx Though the variations between people can be dealt with statistically , the choice of a limiting pressure criterion appropriate to a particular route depends on a number of complex factors .sx The Paper attempts to categorize journeys into four different types , and to allocate a pressure change criterion to each one .sx Such criteria are then used for example by Project planners to size the tunnel cross-sections on the route in conjunction with a route speed profile , or maybe also to consider the need for sealed rolling stock .sx 1 .sx INTRODUCTION .sx A feature of modern rail operations which can cause discomfort to train occupants is the sensation felt on the ears from air pressure pulses propagated by the trains' motion through tunnels .sx The strength of these pressure changes ( see Ref .sx 1 ) increases approximately with the square of train speed , and multiple pulses , of reinforced magnitude , can be produced by the interaction of additional trains in the tunnel .sx Further , the size of the tunnel cross-section area relative to that of the train has a strong bearing on the pressure magnitude .sx Thus , considerable care is needed with the design of tunnels on new routes , especially those for high-speed , to avoid ear discomfort .sx At the present time , a substantial amount of new railway building is taking place around the world , often with considerable lengths of route in tunnel .sx It is the nature of things that the public's expectation of comfort is continuously growing and this emphasizes the importance of choosing the right comfort criterion with which to optimize the tunnel design .sx This Paper is concerned with the questions of how much comfort is expected ( and indeed demanded ) of modern operations , how can a limiting degree of discomfort be gauged and defined in measurable units , and what does this mean in terms of railway operations .sx These questions are difficult to answer .sx Sociological , technical , and economic considerations play an important part .sx The passenger's view of the appropriate standard of comfort together with a railway's preparedness to meet it are largely dependent on the current affluence of the former and the economic well-being of the latter .sx Section 2 of this Paper discusses the comparatively small number of railway pressure comfort criteria that are known to exist at the present time .sx Section 3 describes some of the more important features of human response to pressure change that need to be taken into account in a comfort criterion .sx Section 4 of this Paper examines the range of railway operations that are now demanding an expression of sufficient comfort level , and discusses whether such a universal definition is practicable .sx Section 4 goes on to propose a more flexible approach which allows a limiting pressure specification to be chosen according to the particular circumstances of the rail operation concerned .sx 2 .sx EXISTING CRITERIA .sx The need to fix limit values of pressure change on board rail vehicles for reasons of passenger comfort first arose :sx a ) for high-speed passenger trains operating on new routes in hilly terrain where there were frequent tunnels b ) for new Metro systems where commuter journeys were punctuated at frequent intervals by underground station stops between sections of tight-bore tunnel .sx The common factor between them was the frequent nature of the pressure transients created in the two situations .sx The crucial factors that brought the problem to a head were , in the first instance , high train speed and , in the second , the high blockage ratios of Metro trains within the tunnel cross-section .sx It is useful to examine the form that the existing known criteria take .sx 2.1 Japan Shin Kansen Operations .sx Initial service trials with the prototype 'Bullet Trains' on the newly built Shin Kansen routes in the Sixties drew considerable adverse reaction to the aural discomfort felt by passengers as they passed through the numerous tunnels on these routes .sx By the current standards of the day , the tunnels were built to generous proportions ( 63.4 m 2 cross-section ) in order to alleviate the effect of high speed .sx Nevertheless , the number of tunnels , and consequently the total cost of tunnel construction , limited the adoption of even larger sections to reduce the discomfort to more acceptable proportions .sx As a result of this early experience , Japanese National Railways decided to embark on the revolutionary process of sealing the structure of the passenger coaches so as to prevent , or at least largely attenuate , the pressure transients , produced within the tunnel external to the train , propagating into the coach interiors .sx In spite of the considerable cost and complexity of the modifications to coach construction that this has incurred , JNR and its successor JR have persisted with this design policy right into their latest rolling stock range .sx Having taken the decision to resort to pressure-sealed construction , then it was feasible to go for a high standard of 'pressure' comfort whilst still achieving high train speeds .sx It can also be argued that some of the incurred costs can then be balanced off against the cost savings to the infrastructure allowed by the modest tunnel cross-sections that are feasible .sx As a target standard , JNR have built their Shin Kansen rolling stock to a pressure criterion defined as :sx Max change of pressure = 1000 Pa .sx Max rate of change of pressure = 200 Pa/s .sx However , it is understood that recent reviews of the maintenance costs associated with meeting these standards , particularly as the stock gets older , have persuaded JR to relax their criterion to :sx Max rate of change of pressure = 300 Pa/s .sx It seems that the specification of overall pressure change of 1 kPa ( for which a minimum time scale is only implied ) is left unaltered , although it would appear that this could be infringed in long tunnels .sx The above criteria have been adopted for 'sealed' Shin-Kansen stock , but it is believed that no corresponding criterion exists for Japanese conventional 'unsealed' stock .sx 2.2 US Underground Rapid Transit Systems .sx The American Dept of Transportation ( UMTA ) have produced recommendations ( Ref .sx 2 ) for subway train conditions .sx Subway tunnels tend to be of tight bore and a typical journey involves several tunnel transits as trains pass from tunnel into station and later back into tunnel .sx Thus , the resulting pressure changes are experienced in rather quick succession and all too often by commuter travellers who may be users on a regular daily basis .sx Thus , such travel may demand tight control over pressure variation .sx UMTA recommend a pressure standard which may be expressed as :sx Max pressure change = 700 Pa within a 1.7 s period .sx Max rate of change of pressure = 410 Pa/s .sx ( over periods longer than 1.7 s ) .sx 2.3 British Railways Board .sx In the early-Seventies , a need arose in the UK for a pressure criterion to cater for the new generation of electric trains which were to operate on the newly electrified West Coast Main Line at significantly higher speeds .sx Although this route had relatively few tunnels and these were of the short-to-medium length category , their cross-sectional areas were small ( many less than 40 m 2 ) and thus pressures could be substantial .sx A study of existing conditions , which were apparently acceptable to the public , and of the results of pressure chamber tests with volunteer subjects ( Ref .sx 3 ) resulted in a limiting criterion defined as :sx Max pressure change = 3000 Pa within any 3 s period .sx At the same time , BR adopted a further criterion which was specifically chosen for operation through the Anglo-French Channel Tunnel that was being actively pursued in the early 1970s .sx The configuration of the Tunnel being considered at that time was very similar to the present day Tunnel and concern was expressed at the pressure pulses that would be experienced on the Shuttle Trains as they passed the cross-connecting ducts linking the two separate running tunnels .sx Train operation at 120 km/h meant that these ducts were passed at intervals of approximately 7 seconds , and the British Railways Board ( one of the members of the British Channel Tunnel Company ) adopted a standard for a desirable comfort level during normal operation through the Tunnel which stated :sx Max pressure change = 450 Pa .sx ( when repeated at 7s intervals for the 25 min tunnel journey ) .sx A corresponding standard of 700 Pa was considered to be the upper limit of acceptability for such tunnel operations .sx A rare extreme case was not identified .sx In 1986 , BRB revised their former criterion for inter-city route operation and effectively relaxed the standard to allow :sx Max pressure change = 4000 Pa within any 4 s period .sx Thus , the new criterion allows the maximum short duration pressure change to be 4000 Pa rather than the previous 3000 Pa .sx The basis for the decision was favourable operating experience with the 3000 Pa criterion together with the results of international studies ( Ref .sx 4) .sx The 4000 Pa within 4 s criterion remains the BRB comfort limit for all BR routes except for the proposed Rail Link between London and the Channel Tunnel .sx This route is exceptional within the BR network as it is proposed to have several tunnels ( amounting to approximately 30% of total route length) .sx A recommended pressure criterion for the route has been proposed to be :sx 2.5 kPa within 4 s for single-track tunnels .sx 3.0 kPa within 4 s for double-track tunnels .sx The higher pressure permitted in a double-track tunnel allows for the fact that this limit will only occur when two trains pass at maximum speed within the tunnel and even then only when they pass in a particular place within the tunnel .sx For single-track tunnels , the highest pressure changes felt on each occasion will always be the same ( for a given operating speed) .sx At the time of writing , project studies show that relatively few tunnels are single-track and that therefore this increased level is reasonable , and consistent with route infrastructure economies .sx Notional maximum speed for the route with 'non-sealed' stock is 225 km/h .sx 2.4 The Deutsche Bundesbahn Neubaustrecke Routes .sx In the early-eighties , DB decided to build a network of new high-speed routes which could also be used for mixed traffic operation .sx The first of these routes , from Hannover to Wuerzburg , involved approximately 30% of the route length in tunnel due to topographical and environmental restrictions .sx DB designed the railway for operation at 300 km/h with an ultimate goal in mind of 350 km/h .sx Consequently , large diameter tunnels were planned ( cross-section 82 m 2 ) to limit the pressure problem and to ease traction energy costs associated with aerodynamic drag .sx During pressure comfort tests held in the Derby pressure chamber simulating future NBS operation ( Ref .sx 5 ) with unsealed stock , the senior management of DB came to the decision that they should adopt sealed rolling stock for future high-speed operation .sx It was felt that the superior standards of comfort designed into the NBS operation , with the ICE train in particular , necessitated , at the same time , an improved pressure comfort standard .sx Because of the similarities between NBS operation and Japanese Shin Kansen services , DB decided to adopt the JNR criteria :sx Max change of pressure = 1000 Pa .sx Max rate of change of pressure = 200 Pa/s .sx although it is understood that the 200 Pa/s limit may be relaxed to 300-400 Pa/s .sx 2.5 Others .sx Other high-speed routes such as the Italian Direttissima and French TGV-Atlantique line have multiple tunnels , and presently use 'non-sealed' rolling stock , but FS and SNCF have not declared their limiting pressure criteria .sx However , it is known that the next generation of coaching stock to be used with the new Italian ETR500 train and the proposed TGV-R train will be pressure-sealed .sx There is no standard definition of 'pressure-sealed' stock .sx However pressure sealing has been quantified for test purposes ( see for example Ref .sx 6 ) in relation to the degree of sealing of a railway coach by a time constant associated with the time decay of internal pressure of the coach ( initially at an over-pressure ) after the pressurizing means ( eg ventilation fan ) is removed , according to the law formula .sx where P i = coach internal pressure .sx P I = coach initial internal pressure .sx t = time in seconds from P i = P I .sx tau = time constant in seconds .sx