Obviously, monitors did not prevent the sexual assault in this case. If enhanced DBS is needed to meet the requirements of some of the people involved, I very much doubt whether it is sufficient. As far as I know, this just a system that check people's past criminal records and cannot assess their potential future criminal behaviour.
In this case, only a more complex investigation and evaluation system can predict the employee's future criminal possibility to a certain extent. From the employee's origin: whether he comes from a high-crime area, whether his family members have a criminal history; the employee's personal speech, including social media accounts, is likely to be traced back to their very early records; the employee's family and marital status, such as sexual needs in this case is an important clue.
In theory, these indicators can indicate the crime risk of employees, but this requires a large amount of data aggregation and a large number of professional records and evaluations (You don't want a random person to assess you that "this person looks like a rapist, HIGH RISK"). This requires a large number of investigators, spies, coordination between departments, and a professional assessment team to achieve.
Given the number of railway employees who come into direct contact with passengers, it is clear that a large budget to operate a huge assessment agency is need.