This is not an index of railway staff, but of shareholders in the railway. The Society of Genealogists produced its GWR Shareholders Index from ledgers created by the Great Western Railway and now in the Society's possession. The Great Western Railway's original ledgers were compiled by the company for transactions relating to all shareholdings which changed hands other than by simple sale.

The GWR called the ledgers Probate Books, which reflects the fact that the great majority of such share transfers (approximately 95%) were as a result of the death of a shareholder and their shares changing hands during the administration of the deceased's estate. The proportion of the GWR's total number of shareholders included in the Society of Genealogists GWR Shareholders Index is not known but is estimated to be between 50% and 75%; this is because the railway shares were regarded as gilt-edged stock to be held for the long term.

The index currently contains details for approximately 440,000 individuals, with a total number of 570,464 records and 153,569 events entered into the registers. A record is an entry for an individual in the database, including not just shareholders but executors, beneficiaries and others involved in the transfer of shareholdings. An event is a disposal of a shareholder under a will or an intestacy following death, or otherwise than by simple sale during the lifetime of the shareholder.