In professional driving, maintaining alertness throughout the journey can be challenging. Despite efforts to address decreased alertness, there is no agreed method to investigate mind-wandering in transportation. Trigger-based data collection methods are most common, but can influence participants’ mental condition, reducing mind-wandering occurrences. Besides, in real-life test rides it is not desirable to interrupt and question drivers. Building upon existing research, a mind-wandering investigation protocol and questionnaire were developed to address this. The protocol was tested and refined by querying train drivers about their mind-wandering experiences on a train simulator, allowing drivers to assess mind-wandering across multiple journey phases. The results show that the proposed protocol can provide greater insight into driver alertness and mind-wandering in professional driving settings.