The most optimal and cost-effective storage configuration for this scenario is to configure dual-regional storage with the standard storage class for the dual region closest to the users. This choice ensures high availability and durability by replicating data across two geographically separate regions, which is crucial for a mission-critical application like a train booking system. Standard storage is designed for frequent data access, offering a balance between performance and cost. Storing data close to the users in Bangalore minimizes latency and improves performance. Other options are less suitable because regional storage lacks the high availability of dual-regional storage, and nearline storage, optimized for infrequent access, can incur significant retrieval fees with frequent access.