Be present? Having someone capable to do most basic troubleshooting and mechanic can do being there when it breaks seems quite efficient and much faster response. No travel or alert time. Notify more skilled personnel if found issue not repairable on spot or can't find it.
At the cost of two humans on the train all the time. If that cost isn't greatly higher than the cost of mechanics to go to the train when there is a problem then you have a basic train maintenance issue to solve - or maybe you are running worn out trains that should have been replaced long ago.
