The passenger transportation,safety is more important than efficiency or reliability. Therefore, it is vital to maintain the proper condition of the equipment related to the passengers’ comfort and safety.This manuscriptexplainsthe methodology ofcomplete development and implementation of both hybrid model and digital twin3.0for a HVAC inrailways. The objective of this is to monitor the condition of the HVAC where it matters to the comfort and safety of the passengers in the trains. The level3.0of digital twinwill be develop for the diagnosis and prognosis of HVAC by using the hybrid modeling.The descriptionillustratedin this paper is focused on the methodology used to implement the hybrid model approach and both the need and advantages to use a hybrid model approach instead of thedata-based approach.One of the particularitiesconsidered doing the developmentwas the importanceof the safety and environmental risk whichwere included in the risk quantification of the failure modes. In train companies the maintainers replace critical components in early stages of degradation, thus, using a data-based model might loses important information and does not givegood support to manage the maintenance instructions.Developing a physicsbased-model will be able to generate synthetic data for the behavior of the components in advanced stages of degradation and combiningitwith the data-based modellead to formhybrid model with a large number of failure modes that were unpredictable. Finally,the outcome is beneficialfor the proper functioning of the systems, hence, the safety of the passengers.