# Superinsulation Materials for Energy-Efficient Train Envelopes
###### Abstract  
While traction energy is the main energy used in passenger trains, the energy required to condition the passenger area is significant and can account for up 40% of the total energy use. Due to space constraints, passenger train envelopes are usually only insulated with a few centimetres of conventional insulation materials, leading to relatively high U-values and high energy transmissions in climates where heating or cooling is necessary. Here, the use of superinsulation materials in trains for heating/cooling demand reduction was evaluated. Selected commercially available superinsulation materials were screened and benchmarked in terms of thermal conductivity, fire safety, behaviour under vibration and preferred use position within the train envelope. Both the specific vacuum insulation panels and silica aerogel boards that were tested were found suitable for the application in train envelopes in terms of their thermal, fire and mechanical properties. While vacuum panels have the best insulation performance, aerogel boards offer higher flexibility during installation and reduced risk of damage-related performance loss. Both materials can greatly enhance the energy efficiency and thermal comfort of trains and can be implemented cost-neutrally. However, new policy frameworks are likely to be necessary to promote energy efficiency and the broad implementation of thermal superinsulation in trains. a 2021 11 2939  
Through both lab and field studies, the present work aims to answer the question of whether, and how, thermal superinsulation materials can be used in passenger train envelopes in order to improve the train's energy efficiency. To give context to this question, the energy use in passenger trains and the corresponding saving potential will be reviewed, followed by an overview on the thermal envelope of trains as well as insulation materials in trains. Subsequently, a description of thermal superinsulation materials and their applications in buildings as well as examples of superinsulation materials in transport will be given. Traction energy, i.e., the energy needed to move the train, is the main energy use in passenger trains. Traction energy use and its optimisation have been studied thoroughly. For example, [PERSON] and co-workers [1] modelled the overall energy and greenhouse gas impact of high-speed trains (including manufacturing and infrastructure) and found values between 54 and 178 g of CO\({}_{2}\)-equivalent per passenger kilomet travelled, which is significantly lower than for air or car travel. The actual traction energy can oftentimes be optimised further. [PERSON] et al. [2] found that timetable optimisation can lead to reduced travel times in urban trains and a total energy use reduction of up to 27%, whereas [PERSON] and co-authors [3] found an almost 8% improvement potential in timetable optimisation for a high-speed train. Additionally, maximum urban train speeds can be optimised for energy use as demonstrated by a study of [PERSON] and colleagues [4]. While traction energy use is the largest part of the total energy needed to operate the train, the energy necessary to condition the train's interior, i.e., the energy need for heating, ventilation and cooling (HVAC), can be significant, accounting for up to 30-40% of the total energy use [5; 6]. Hence, the optimisation of HVAC energy use is an evident next step after considering traction energy in order to make railway transport even more sustainable. [PERSON] and co-workers [7] used a model predictive control-based framework to optimise HVAC operation in trans, which was implemented in a tram in Vienna, calculating an estimated annual energy saving of 32%. By modelling the conditioning of a Swiss train of Rhatische Bahn with the software IDA ICE, [PERSON] and co-workers [8] evaluated the saving potential of different improvement measures in terms of HVAC operation, train envelope and HVAC components that would keep passenger comfort unaffected.