If she were on the opposite side how easy to slip her hand through the opening and turn the handle of the door.

But this was impossible.

She could not hope to succeed.
The other thing which fixed itself in her mind was the familiar tone of the coachman towards Dorrimore.

It was more that of an equal than of a menial.

This impression confirmed her suspicion that she was trapped.
Dorrimore had doubtless enlisted the services of a confidential friend rather than trust to a servant whose blabbing tongue might serve to betray him.
Meanwhile Dorrimore's head was still out of the window.