I'm not aware of any credible evidence that as a general principle that monitoring workers reduces ability to perform tasks.
"Complex" is the key term. Generally speaking, there is an optimal level of psychological arousal for performing given tasks. For tasks that are simple, rote, and/or well-learned, that level is higher than it is for tasks that are difficult or novel. In the specific case of knowing that you are being observed, it tends to decrease performance on difficult tasks and have varying results on simple tasks. See Social facilitation.
The question here is whether or not the job of train operator qualifies as simple and rote, or difficult. I could easily see it being the former, where the tasks are not difficult, and the challenge is to maintain attention or vigilance. If that is the case, then the awareness of being monitored could well improve performance in itself.