Last time I was on one, it was via the test engineers laptop.
There was suggestion that it will be done by gps for now, obviously ETRMS lines can use this instead.
The engines preheat and start in advance and from what I remember of the manuals I read through the train treats it as if it is entering a neutral section as the vcb opens, the pan drops and the aux power supply is brought online, supplied by the diesels.
...in theory.