Hi Richard,
This issue is neither PC or Mac.
When you place consists on your route, there are AI drivers assigned to the locomotives by default. You can control this by doing two things, and actually in two different ways.
1) Place the consists like you normally do, which is the way you are doing now.
2) Edit the Session and edit the session setup. On the interface, this is the icon on the menu bar to the right of the filter - it looks like a piece of paper for a lack of another description.
3) In the list edit the driver commands.
You will see a bar next to the driver's picture and name as they are assigned to each drivable consist.
4) Click on the driver and click on remove.
This will leave the consist, but remove the driver which you want, I think.
5) Click add driver on a blank line and add a driver and repeat three or four times to add in unassigned drivers.
The purpose of this is to allow you to assign drivers to these trains at your whim while operating the session. You can then click on the driver list (on the PC it's the 2 key and then clicking on the driver picture to raise it up so the whole list is showing), pick a driver and choose move-to-train. The train being the one you want the AI to drive.
The other way is to place your consists as described, then use the Quick Drive menu to add drivers as needed.
I have done this for years as I have created driver sessions with some having many drivers assigned with a handful of others waiting on the roster for assignment. During my operation, I may need an extra driver to work a yard, for example, and once a driver is assigned to a consist I can then take control of it myself. At any rate, this I think will help you achieve what you want.
John