# Full-Scale Experimental Investigation of the Interaction between Trains and Tunnels
## 3 On-Board Tests
### Inner Pressure  
Up to now the pressures generated by the passage of a train in a tunnel was analyzed from the point of view of both the track and the train, but always referring to the outside of the train. In this section, however, the pressure changings within the two types of train will be shown. While outside the train there are not many differences in the two cases, apart from those depending on the speeds, inside the train the behaviors are completely different. If on the one hand the sealing increases the comfort of the passengers, on the other the high-pressure differences between the internal and external surfaces generate relevant loads on the structure. As regards the unsealed trains, the external pressure is less filtered and, therefore, the on-board comfort is, in general, worse. The advantage is that they do not have to withstand big static loads. However, the dynamic loads at a higher frequency are not filtered and they can lead to fatigue problems over time. On the other hand, the unsealed trains usually travel at lower speeds and, consequently, the self-generated loads are also lower. The problem arises in the case of mixed traffic, i.e., when an unsealed train travels on a HS line crossing HS trains. Figure 15a reports the comparison between various inner pressures in different points of the sealed train. It can be clearly seen that in the case of a sealed train the internal pressure is practically equal throughout the train and the whole train is sealed with respect to the outside but not internally between different cars. Since the small openings towards the outside are distributed throughout the train and the pressure information travel at the speed of sound, the differences are not appreciable inside. For comparison, an example of pressures measured inside the unsealed train is presented in Figure 15b. As the train is less filtered, in this case the inner pressures follow the trend of the corresponding external pressures and, since the passage from one vehicle to another is practically open to the outside, the pressure within each vehicle is not influenced by that of other vehicles. To highlight the differences, in Figure 16 a crossing between the unsealed train and an HS train running at 250 km/h is considered. In this case the first order system response of the inner pressure and the differences between different carriages are clearly visible. On the other hand, inside a car the pressure is uniformly distributed, as proven by the sensor in the middle of the first car and the one in the cabin. Figure 14: Comparison between the pressure acquired in different sides of the unsealed train in case of crossing within a tunnel. In both cases the internal pressure evolves from the external one, roughly following the first order system described by Equation (1) (an advanced model could be used, also considering the sign of the \(\Delta P\)[66]). Using this model, the time constants \(\tau_{\text{dyn}}\) an be estimated by means of a least squares method by comparing the calculated internal pressure with the measured one. Pressurized Trains  
To be thorough, but only for information purposes, it has to be said that the sealed trains are also pressurized. The HVAC (Heating, Ventilation and Air Conditioning) system is equipped with shutters that can open and close automatically, relying on external pressure signals. During a run in the open field, the shutters are opened so that the system can suck in air from outside, keeping the internal  
Figure 16: Comparison between the pressure acquired in different positions along the unsealed train outside and inside in case of crossing within a tunnel. Figure 15: Comparison between the pressure acquired in different positions, outside and inside, along the sealed train (**a**) and the unsealed train (**b**). pressure of the train about 200-300 Pa higher than the environmental one.