] .... pinned up each other's train for the dance

] Does anyone have any insight into the meaning of that bolded phrase? ..............Can we use this fashion comment in order to date the work to 1798, or would it be more common later, nearer the time of publication?

Train pinning was a practice that had gone on for years. It was usually a subserviant thing to do, so to pin one another's train shows equity and also a lack of paid help. The best example from JA's family is that of JA's great, great grandmother, Joanna Pury (born about 1700), married to her great great grandfather, Lord William Leigh. Joanna (and her sister) had a maid when they were young who later became a Duchess as she married General Monk (he became Duke of Albemarle).

There is a story that both ladies were (I think at court) waiting to be presented and the Duchess of Albemarle was helping Lady Leigh adjust her train and made the remark that she had formerly been employed to pin it up. (From Helene Leigh, "Stoneleigh Abbey Pictures")