The barrier-free expansion of public transport is a pressing issue that sometimes calls for individual surveying solutions. Especially at subway stations that are more than 100 years old. This project aims the development of a measuring system to determine the gap width between a train and a platform after the platform was rebuilt on the same level of the train doors. The measuring system is used to determine the gap-filler size to reduce the gap between train and platform. In a follow-up project phase, the sensor system was modified into a monitoring system with regard to detect the contact of trains with the gap-filler. The developed sensor system consists of MEMS sensors. This project was honoured with the 3. place of DVW Best Practice Award 2017.