Usability Evaluation – User Experience Indonesia Train Ticket Reservation: System Usability Scale (SUS) and Think-Aloud Protocol

Public transportation applications play a crucial role in digital mobility, yet their adoption strongly depends on usability and user-centered interaction design. Although KAI Access is widely used in Indonesia, recurring issues such as login failures, transaction errors, and system delays indicate gaps in its Human–Computer Interaction (HCI) design, particularly in feedback mechanisms, system reliability, and cognitive workload. This study evaluates the usability of KAI Access and identifies key interaction factors affecting user satisfaction. We applied the System Usability Scale (SUS), Think Aloud Protocol (TAP), and the ISO 9241-11 framework to assess effectiveness, efficiency, and satisfaction. A total of 114 respondents completed the SUS questionnaire, and 30 participants performed TAP across five booking scenarios. Results showed a marginally acceptable SUS score of 64.32 and a task success rate of 71%, with delays, unclear system feedback, and booking errors emerging as primary usability barriers, though users appreciated interface simplicity. These findings underscore the need to enhance system responsiveness, strengthen error prevention, and refine interaction flows. Theoretically, the study contributes to HCI by emphasizing how system performance, clarity of feedback, and reduced cognitive load shape mobile travel app usability. Practically, the insights offer concrete directions for improving login, booking, payment, e-ticket, and post-purchase processes to support KAI Access’s future development roadmap.