Whenever i catch a train From St Pancras In't To Faversham at Ebbsfleet Domestic the AI trains do not switch to 3rd rail it's the same with Ashford In't they don't switch to 3rd rail i believe this can be a bug so hope this can be fixed.
If my memory serves well, you have to do that manually. There´s a key combination on your left, switching from DC to AC and vice versa, shoes and pantograph up and down. For the correct sequence, consult the tutorial. I´m to lazy to produce screenshots for that matter.