According to the in game control guide, holding square in external view should activate new controls for the external camera such as the ability to cycle through cameras and zoom in and out, however this does not work and instead controls the train. I believe this may be a bug so hopefully this can get fixed ASAP.
OK so quick update, I just realised R3 makes it work, not square. So maybe change the control guide DTG.
Where is the control guide? I looked a bit and couldn't find one. Also, do you know if there's a key mapped for the ACS acknowledge? Usually O takes care of all alerts, but it's not pressing the ACS button.
I think the control guide is accessed from one of the tabs along the top that you gat after you press options. I also think it is still the O as Matt said they didn't want to move anything people are use to. Maybe it is shift O now, square plus O