Can't Set Player Train In Build-created Scenario - Donner Pass Ts2019

Hi TS2019 community -
We are trying to create a custom scenario based on the Donner Pass DLC. For some reason the box to click to make the train player-driveable (as opposed to autorun) is greyed out (present but not clickable). How can we switch this toggle?
Welcome any suggestions here.

 

Is the loco Driveable? If you have a loco which is designed for AI, without cab controls, you get this error.
Are you creating a Standard or Career Scenario as that might stop you changing the parameters of the Scenario
And is the Scenario unlocked?

Peter