This graduation project makes a contribution to this complex challenge by researching the question “How can NS achieve circular inflow and outflow during building, modernisation and end-of-life for the train interior, car body and bogies?”. A contribution to this challenge was made by creating a design concept for a sustainable interior side wall panel. The current panel is made of glass fibre polyester composite, which is difficult to reuse or repurpose, and cannot be recycled. The sustainable alternative is made use of recyclable aluminium honeycomb materials, and has a dismountable structure due to the use of reversible Niaga adhesive. The flat shape of the panel combined with the optimized surface distribution increase the available panel size, which increases the potential for reuse or repurpose. Additionally, the wall panel is finished through the use of coloured foil instead of paint as this reduces toxic substances and improves cleanability, readjustability and recyclability. It also gives additional customization options to the panel, such as integrating a honeycomb pattern to illustrate the circular construction. This report also reflects on the application of design methodology by researching the question “How can design methodology be used to structure a complex design project?”. Two design methods were chosen for this: the 1:10:100 approach and the Design Roadmapping methodology. The 1:10:100 approach was chosen as this is suitable for open-ended projects. I found that the 1:10:100 method was very useful to quickly determine a project scope but it lacked structure within the ‘100’ cycle. I found the midterm and green light graduation deadlines to be useful additions to further structure the iterative process. Design Roadmapping was chosen as it combines future roadmapping and design. NS was interested in creating a roadmap towards fully circular trains, whereas the Industrial Design Engineering department was interested in my skills as concept designer. The roadmap has not been finished within the timespan of this project, but this report makes good progress along the analyses steps of this method. Additionally, I felt this method helped me to keep focus on the future instead of focusing too much on current practice.