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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.

Silica aerogels display an ultra-low thermal conductivity (λ) and are used as thermal superinsulators. Here, we study the influence of aging and drying processes on the microstructure and thermal conductivity of fiber-reinforced silica aerogel composites. Glass wool-silica gel composites were aged for variable times, hydrophobized, and dried either at ambient pressure or from supercritical CO2 (scCO2). The X-ray micro- tomographic data display three distinct phases: silica aerogel, glass fibers, and macroscopic pores and cracks. The silica aerogel appears as a continuous medium in the tomograms because the spatial resolution (6–11 μm) is insufficient to resolve the aerogel mesopores (∼0.02–0.10 μm). For the composites prepared by ambient pressure drying, insufficient aging led to prominent drying shrinkage and cracking, and a high macro-porosity, as quantified by 3D image analysis. Insufficient aging also led to an increase in λ from 15.7 to 21.5 mW m−1 K−1. On the contrary, composites that were nearly free of cracks and displayed a constant λ of 16.3 ± 0.8 mW m−1 K−1 could be prepared by scCO2, independent of aging time. The thermal conductivity was reproduced from the macro-porosity to within 0.7 mW m−1 K−1 using simple thermal transport models consisting of thermal elements connected in series or parallel. Our results illustrate the usefulness of X-ray micro-tomography to quantify the 3D microstructure and its effects on the bulk composite properties and the data highlight the importance of aging for the production of low λ aerogel-fiber composites by ambient pressure drying.