Following up from some questions from a recent post on Coachmen, I decided to dedicate an entire explainer to the word apron and how it relates to coaches and coachmen. The below gives a thorough overview of the different types of apron and what they are made out of. This modern site has some examples of different types of aprons now available that are illustrative of the general style that would have been relevant.
Essentially, the apron functions like a lap robe, but as it is fastened to the body with a strap and sometimes a buckle, it is widely referred to as an apron. The apron also helped to keep other passengers warm as explained below, and was part of the reference in the recent Coachmen post.
Thank you. I had wondered, too.
You are so welcome, Susan!