**1. Introduction**

World population is increasing, and therefore the needs for mass transportation. Nowadays, railways are one of the most widely used public transportation systems, mainly because of their safety, superior transportation capacity, reduced boarding time, and the possibility of making better use of the journey time to work or enjoy train facilities. Additionally, trains have been revealed to be the most sustainable transportation system. In Europe, the predominant proportion of rail services are operated by electrified trains; thus, its CO2 emissions are residual. The emissions per passenger-kilometer are much lower for the train than for air or car transportation [1–3].

Due to its low environmental impact, multiple governments are promoting rail use as the major mass transportation system, especially for middle-range distances. Based on this proposal, train passengers have continuously increased from 2013 until the

2020 COVID pandemic. Between 2015 and 2019, a historic 4000 billion passengerkilometers were recorded worldwide. However, during the pandemic years, due to multiple lockdowns, train passenger numbers drastically decreased [4–6]. Currently, those numbers are even higher than the pre-pandemic ones [7].

The quality of public transportation services influences travelers' choices. Passengers with previous good travel experiences will probably use the same travel transportation mode again. On the other hand, customers that experienced problems with the journey service may change to a different transportation mode for their next trip. Therefore, to keep increasing continuously the number of passengers, it is crucial to raise trains' attractiveness and provide comfortable journeys [8, 9]. Those are defined based on safety, comfort, and user conditions [10, 11]. In particular, the seat and vibration are strongly linked with discomfort. Passengers spend most of their travel time seated; thus, vibration, derived from train motion and train-rail interaction, is transmitted to the user through this surface, being classified as whole-body vibration (WBV). A specific feature of a seat is its dimensions which must be able to accommodate people with different anthropometric characteristics, and simultaneously provide low fatigue levels and a general feeling of comfort [12]. Although affecting passengers' comfort, vibration is also linked to safety. High safety levels are ensured through adequate maintenance. That will guarantee the reliability and longevity of the rail, which needs to provide a stable and safe platform for train operation. Corrective and preventive interventions are accomplished during track maintenance. Its main goal is to preserve the system's functions and prevent its breakdown or failure [13–16].

Comfort and discomfort are two concepts not consensually defined in the literature. Hence multiple interpretations can be found. However, it is well established that comfort should be evaluated based on objective and subjective methods, ideally a combination of both. Objective methods are derived from mechanical approaches; thus, these methods quantitatively define the physical variables of comfort [17]. In opposition, subjective evaluation methods rate users' feelings and, this way, they quantify the psychological impact of comfort on the passengers based on questionnaires and rating scales [18].

Comfort parameters are divided into "Motion" and "Non-Motion" factors. The latter is characterized by issues such as noise, smell, illumination, humidity, and temperature. To properly classify "Motion" parameters, it is essential to conduct both dynamic (in the presence of vibration) and static (absence of vibration) tests. The former is evaluated by ride comfort, seat effective transmissibility (SEAT) and transmissibility tests. The latter is mainly defined by interface pressure measurements [10]. Seats may present good dynamic behavior but poor static performance. The ideal seat has a combination of optimum dynamic properties to minimize unwanted vibration and the best static behavior to equally distribute pressure at the seat surface and, this way, reduce the interface pressure.

The present chapter intends to introduce a new methodology to identify train and rail track infrastructure sections' maintenance requirements based on ride comfort analysis. Moreover, transmissibility and interface pressure experiments are conducted as complementary comfort analysis methods. Vibration transmissibility allows the study of the vibration frequency that is being transmitted to the user. If associated in combination with the interface pressure (static analysis), it is possible to define the complete seat structure and conclude about passengers' comfort. The chapter is organized into seven sections to report on the aforementioned goals. Multiple comfort and discomfort definitions will be presented, and the one used in this research will be detailed. Based on vibration analysis, the ride quality evaluation methods are defined, leading to the rail vehicle and track infrastructure maintenance needs identification. Finally, transmissibility and interface pressure experiments are described leading to the final passengers' comfort evaluation and seat structure analysis.
