In the event of a failure the train driver does some troubleshooting steps (like rebooting the control system or manual overrides) and informs the central of the issue. The most common issue on train service are door malfunctions, which can be solved by locking the door and putting a sticker on it.

The difficult part is recognising the problem correctly, which probably requires some critical thinking skills. But the control systems are getting better at helping the driver and giving information on the issue.

I imagine it's learning the signalling and safety systems, the driving dynamics of trains, getting to know the lines and schedules and the corresponding train models. And then learning and practicing driving with a trainer. Also basic troubleshooting for technical problems.