From November, 2006 to March, 2008 a series of tests were performed onboard a wide variety of trains in order to check their response to pressure waves generated while passing through tunnels. In this communication part of the experimental results are presented, showing the pressure waves generated and focusing on the differences caused by some parameters involved such as train length and shape or tunnel lengths. The results are in accordance with the train wave signature method and the one-dimensional pressure wave theory

Gaite Cuesta, José

Martínez Muelas, Alejandro

Meseguer Ruiz, José

Vega Ramiro, Enrique

English

Archivo Digital UPM

A wave signature based method for the prediction of pressure transients in railway tunnels,

The alleviation of the aerodynamic drag and wave effects on highspeed train in very long tunnels,

Pressure measurements on real high-speed trains travelling through tunnels

Servicio de Coordinación de Bibliotecas de la Universidad Politécnica de Madrid

 1
BBAA VI International Colloquium on: 
Bluff Bodies Aerodynamics & Applications 
Milano, Italy, July, 20-24 2008 
PRESSURE MEASUREMENTS ON REAL HIGH-SPEED TRAINS 
TRAVELLING THROUGH TUNNELS 
Alejandro Martínez, Enrique Vega, José Gaite and José Meseguer 
IDR/UPM, E.T.S.I. Aeronáuticos  
Universidad Politécnica de Madrid, E-28040 Madrid, Spain  
e-mails: alejandro.martinez@upm.es. e.vega@upm.es, jose.gaite@upm.es, 
j.meseguer@upm.es 
 
Keywords: Train, pressure distribution, tunnel. 
Abstract. From November, 2006 to March, 2008 a series of tests were performed onboard a 
wide variety of trains in order to check their response to pressure waves generated while 
passing through tunnels. In this communication part of the experimental results are presented, 
showing the pressure waves generated and focusing on the differences caused by some 
parameters involved such as train length and shape or tunnel lengths. The results are in 
accordance with the train wave signature method and the one-dimensional pressure wave 
theory. 
Martinez, Vega, Gaite and Meseguer 
 2
1 INTRODUCTION 
In the last years a deep transformation of the Spanish railway transportation system is 
being produced due to the large increase of high-speed railway lines. This has given rise to a 
flourishing period of innovation in both high-speed infrastructure and vehicle technology. In 
this frame of vehicles travelling at speeds as high as 300 km/h, or even larger, aerodynamic 
effects play a role in the vehicle operation, and so aerodynamic problems have focused the 
attention of many scientists. 
Research efforts have been significantly directed to the design of high-speed trains running 
through tunnels because of the need to travel at high velocity notwithstanding the presence of 
geographical obstacles, such as mountains or rivers. Issues related to the aerodynamics in 
open spaces become even more involved when the vehicle runs through a gallery, since 
compression and expansion waves are generated when the train passes an opening or 
encounters a change in the tunnel cross-section. These waves produce discomfort for the 
passengers and give rise to further complications such as the possible damage of the vehicle 
and the release of spherical micro-pressure waves from the tunnel apertures, which causes 
environmental disturbance. 
The aerodynamic problems occurring when a train travels at high speed in a tunnel are 
more complicated than the ones arising in open air travel. The aerodynamic drag and pressure 
loads on the train are strongly dependent on the pressure waves in the tunnel. Aerodynamic 
drag travelling in a tunnel can significantly increase, compared with that in open air. When a 
high-speed train enters a tunnel, a compression wave is formed ahead of the train which 
propagates along the tunnel at nearly sonic speed (in the same way, an expansion wave is 
formed at the train end). When these finite amplitude pressure waves reach the tunnel exit, the 
compression wave is partly reflected back into the tunnel, forming an expansion wave, and is 
partly emitted outside as a micro-wave. A complex wave interaction occurs inside the tunnel 
due to successive reflections of the pressure waves at the exit from and the entry to the tunnel. 
So the train meets the waves several times during its passage in the tunnel. When combined, 
the waves can cause relevant aerodynamic loads on the vehicle and tunnel structure, and can 
also affect the passenger’s comfort, when the train air tightness is not enough to attenuate the 
transmission of the pressure waves into the vehicles. 
A number of papers dealing with such phenomena can be found in the literature, the 
problem being extensively studied both theoretically and experimentally. From the theoretical 
point of view the aerodynamics of a train travelling inside a tunnel (the aerodynamics of 
train/tunnel systems) is governed by a three-dimensional, unsteady, turbulent, compressible 
flow. However some attempts have been made to develop simple one-dimensional models 
which in many cases provide sufficiently accurate explanations of experimental data. On the 
experimental side, many laboratory tests have been performed to analyse the influence of a 
wide range of parameters on the waves appearing inside the tunnel (most of these parameters 
being concerned with the tunnel entrance geometry). Measurements in real trains, although 
they have been done, are scarce, probably because of the economic cost associated with 
experimentation with real train vehicles. 
Martinez, Vega, Gaite and Meseguer 
 3
2 THE TESTS 
From November, 2006 to March, 2008 series of tests were conducted onboard a wide range 
of trains. The main goal of these tests was to validate the capability of the air conditioning 
systems and the air tightness of the trains to cope with the variations of pressure generated 
when the trains entered into and travelled through a tunnel. The measures presented in this 
paper were taken with the aim of understanding the internal variations of pressure and not as 
the primary result of the tests. 
The tunnels that were most interesting and posed most problems were located between the 
towns of Sagides (159 km away from Madrid) and Ricla (250 km away from Madrid) in the 
railway to Zaragoza. 
In order to achieve the speed needed and to be free to pass as many times as possible 
through the same tunnels, tests were performed during the weekend nights, starting at 23.00 
and concluding at 06.00. In these conditions the railway was assigned to the train, so the tests 
could be conducted with almost no interference from the traffic control. As a result, a 
minimum of two passes were performed through each of the sixteen tunnels, providing a large 
amount of data.Table (1) shows the tunnels in which these tests were performed. 
Tunnel Length (m) Area (m2) 
Sagides 1786 100 
Alhama 630 110 
Bubierca 2433 80 
Las Dehesillas 861 115 
Castejón 392 100 
Ateca 464 100 
La Almunia 1014 100 
Marivella 620 110 
Paracuellos 4784 75 
Sabiñan 554 110 
Purroy 847 110 
Las Minas 302 95 
Villanueva de Jalón 1009 100 
Torrecilla 888 110 
Los Cortados 325 95 
Las Calesas 120 95 
Table 1: Tunnels in which tests were performed. 
Being the goal to check the comfort of the passengers, it was a requisite of the train line 
operator RENFE to test all the trains that were suitable for the line, and at a later stage the 
“Séneca” laboratory train was added. This is a very special train, since it is commonly used 
for studying the line and it is a very short train but with high speed capability. Furthermore, 
the train’s motor coach geometry is almost identical to that of other trains in service at the 
moment. In Table (2) a list of characteristics of the trains that were studied is presented. 
Martinez, Vega, Gaite and Meseguer 
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Train Length (m) Top speed (km/h) 
Train A 201 350 
Train B 200 350 
Train C 107 250 
Train D 250 230 
Train E 107 250 
Train F 48 160 
Train G 180 250 
Train H 85 350 
Table 2: Characteristics of trains involved in the tests. 
3 INSTRUMENTATION USED AND DATA CONDITIONING 
The instrumentation consisted mainly on several pressure sensors. Some of them were 
installed to measure the internal pressure and are not of this paper’s interest. The rest were 
installed to sense the static pressure in the outside of several parts of the train, and its 
measures are discussed in this communication. 
The equipments used during the test were conditioned by the fact that the train availability 
was very short, since all the instrumentation had to be mounted, used and dismantled during 
one night. 
Being the results of the tests depending on the air tightness, sealing had to be kept, and so 
the instrumentation was designed accordingly. In order to get the pressure signals from the 
outside, false button plates were installed at the doors with 8 mm pressure taps. In some cases 
the static pressure port of the train was used, placing the sensors in parallel to those belonging 
to the train. 
The instrumentation used for each train was slightly different, due to the requisites of each 
train manufacturer, but the backbone of the equipment was almost the same. 
The sensors were installed in pairs in three sections of the train, namely the head, a point 
near the centre and the tail. Each pair consisted on a internal pressure sensor and an external 
pressure sensor. Measurements were made during passes in both directions, so the 
instrumentation placed at the motor coaches behaved sometimes as “head” and sometimes as 
“tail”. Sensors in both motor coaches of the train were placed at a distance of nearly 10 metres 
away from the train ends. 
Electric noise was one of the first concerns, due to the fact that signal wires would have to 
go across the electric equipment rooms of the trains and extend for as long as 100 metres. 
Pressure sensors with a standard 4-20 milliamp current output were selected to reduce the 
noise and also high-quality shielded cable was used. 
The foreseen high speed of the studied phenomena imposed the need for piezo-electric 
transmitters with one millisecond response time. 
Altitude change effects were of the same order as the expected pressure variations. To 
prevent the saturation of the sensors, electrically operated valves were installed, so the 
pressure was equalized on both sides of the sensors prior to the entry into the tunnel. A few 
seconds before the entry, the reference port was closed, and the pressure measured was almost 
zero at the entrance of the tunnel. This allowed the use of differential pressure sensors with a 
range of only ±7000 Pa in most cases. 
Martinez, Vega, Gaite and Meseguer 
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Even with the valves installed, in several cases the variations caused by the altitude change 
are of the same order as the variations caused by the interaction between train and tunnel. This 
can be seen in Fig. (1). 
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Figure 1: Pressure variations at Sagides tunnel. Train speed 245 km/h. Measurements from the head of the train 
during the whole pass through the tunnel 
In order to get a good synchronization of the samples all the signals were collected by a 
single central acquiring device, a National Instruments CompactDaq chassis with two analog 
to digital modules, each one capable of acquiring four channels. A sampling frequency of 100 
Hz proved to be appropriate, showing the effects with sufficient detail while keeping a 
contained size of the data file. 
The acquisition modules used were two National Instruments 9215 with 16 bit resolution 
and the capability of measuring four channels simultaneously. Precision resistors were 
installed to convert the milliamp signal to a voltage signal. 
Data was collected without any filtering and Butterworth low-pass filters were applied later 
to the acquired signals. 
Additionally to the pressure signals, the temperature was monitored and in several cases 
the command signals to the air conditioning system were also acquired. 
To protect the equipment from the low quality of the electric power available in the trains 
and to cover the neutral zones, a UPS was installed. 
Martinez, Vega, Gaite and Meseguer 
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4 ANALYSIS OF THE RESULTS. 
4.1 Different tunnel Lengths.  
When a train enters a tunnel, it generates a set of pressure waves that are reflected at the 
end of the tunnel in the form of expansion and compression waves. This process is repeated 
several times and so the train encounters the reflection of the waves that the train itself has 
generated. The length of the tunnel is the main factor that determines the time elapsed 
between the entry of the train and the moment of the first and successive reflected wave 
encounters. The shape of the pressure variations is, then, almost the same while the time 
elapsed between those variations depends on the tunnel length. 
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Figure 2: Pressure variations at the tunnels of Sagides (1786 m), Bubierca (2433 m) and Paracuellos (4784 m). 
Train speed from 285 km/h (Sagides) to 300 km/h. (Bubierca and Paracuellos). Measurements from the head of 
the train B during the whole pass through the tunnel. 
Martinez, Vega, Gaite and Meseguer 
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4.2 Different trains passing through the same tunnel.  
The train speed is the key factor for the wave intensity. In Fig. (3) we can see the effects on 
several trains with different speeds. Also, as the train stays longer in the tunnel, more 
reflected waves have time enough to reach the train. To see this increase in the number of 
waves, the train has to stay into the tunnel more than a certain critical time. 
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Figure 3: Pressure variations at Paracuellos tunnel generated by several trains. Train speed varying between 220 
and 300 km/h. Data from the center sensor. 
Martinez, Vega, Gaite and Meseguer 
 8
4.3 Different train lengths.  
During the tests we had the opportunity to compare two different trains with the same 
shape but very different lengths. The main differences were found in the pressure variations in 
the head at the entry into the tunnel. As we can see, the overpressure builds up in a very 
similar way, but the expansion wave generated by the tail reaches the head of the shorter train 
earlier than the longer one’s. 
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Train H
 
Figure 4: Pressure variations at the entry of Paracuellos tunnel generated by two trains with similar shape and 
different length. Train B is 200 m long, train H is 85 m long. Speed 300 km/h. Data from the head sensor. 
4.4 Different train shape.  
While the shape of the train head does not seem to determine the values of the pressure 
variations for a given speed, it seems to determine the way in which the overpressure builds. 
In the case shown in Fig. (5) both trains were travelling at 300 km/h. Both trains have similar 
length and cross-section. The larger amount of noise seen in the graph for the train marked in 
red can be of either aerodynamic or electrical origin. 
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Figure 5: Pressure variations at the entry of Paracuellos tunnel generated by two trains. Data from the head 
sensor. Train speed 300 km/h 
Martinez, Vega, Gaite and Meseguer 
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4.5 Pressure variations at different sections of the train  
As can be seen in Fig. (6) different sections of the same train undergo different variations 
of pressure. While the head causes a compression wave and receives an overpressure as high 
as 800 Pa, in the rear section there is no overpressure, and an expansion wave is created. 
Similar results are obtained in all the trains under study, being the explanation quite simple: 
While the head of the train encounters still air, the tail encounters air in motion accelerated by 
the viscous forces established between the train and the tunnel. The deeper under pressure is 
explained by the higher velocity of the air in the tail of the train. 
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Figure 6: Pressure variations at several sections of the train. Entry of the Paracuellos tunnel onboard train A at an 
approximated speed of 300 km/h 
Martinez, Vega, Gaite and Meseguer 
 10
5 COMPARISON WITH PREVIOUS THEORIES  
When a train enters a tunnel, its head generates a compression wave. This wave propagates 
along the tunnel at nearly sonic speed. In the same way, an expansion wave is formed at the 
train end and propagates at the same speed. These waves are depicted in Fig. (7) as CW 
(compression wave, in green) and EW (expansion wave, in blue). 
The finite amplitude pressure waves reach the tunnel exit, and then the compression wave 
is partly reflected back into the tunnel, forming an expansion wave, and is partly emitted 
outside as a micro-wave. Correspondingly, the expansion wave is reflected as a compression 
wave. 
In the upper part of Fig. (7) the pressure variation for train B while crossing Paracuellos 
tunnel is shown. In the lower part a scheme shows the pressure waves generated, being the 
vertical axis the distance traced adimensionalised with the total length of the tunnel. 
The pressure changes that the train undergoes are generated by the encounter with the 
pressure waves that the train itself has generated. Then, in the figure, the pressure changes 
correspond to the crossing of the lines representing the waves and the position of the train. 
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Figure 7: Comparison between pressure wave theory and experimental data from Paracuellos tunnel onboard 
train B at an approximated speed of 300 km/h 
Martinez, Vega, Gaite and Meseguer 
 11
The measurements presented in this communication are in accordance with the train wave 
signature and the pressure wave theories, summarized in Fig. (7) but also show a greater detail 
in certain aspects. For example, the shape of the first compression wave is clearly different 
between the trains under study, a detail that cannot be deduced from any of the theories 
mentioned. These details, and also the influence of the tunnel geometry or section and many 
other parameters involved are to be studied deeper in the future. The processing and 
understanding of large amounts of data such as the ones briefly presented in this paper is a 
long term and time-consuming task that has to be taken with care and patience in order to 
extract the high quantity of information contained inside. 
REFERENCES  
[1] M. William-Louis & c. Tournier. A wave signature based method for the prediction of 
pressure transients in railway tunnels, Journal of Wind Engineering and Industrial 
Aerodynamics, 93, 521-531, 2005. 
[2] A. Baron, M. Mossi & S. Sibilla. The alleviation of the aerodynamic drag and wave 
effects on highspeed train in very long tunnels, Journal of Wind Engineering and 
Industrial Aerodynamics, 89, 365–401, 2001. 
 


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Pressure measurements on real high-speed trains travelling through tunnels

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