In the context of Onboard Industrial Control Systems (OICS) in the Transport sector, cybersecurity standards lack specificities for offline connectivity periods. This master thesis addresses these challenges highlighting the need for a hybrid design of centralized landside support security system (LSSS) for security and fleet management. Using the IEC 62443 framework, we identified factors impacting Security Level Target (SL-T) estimation, such as evolving threat actor landscapes and physical protection. We analyzed the mandatory and recommended IEC 62443 requirements for integrating LSSS functions and opened discussions for reducing the SL-C of critical OICS requirements and the role of nation-state backed threat actors. Our approach incorporates NIST SP 800-53 controls and other technological references, offering implementation guidance. The proposed hybrid design significantly reduced maintenance costs by 2.5 times while maintaining security robustness, enhancing both cybersecurity and maintainability for transportation sector fleets.