There have been two cargo steamers named Deal belonging to the railway companies over the years.
The first was originally the London, Brighton and South Coast Railway cargo steamer Prince Arthur of 1896. She was sold in 1901 to the South Eastern and Chatham Railway, renamed Deal, and scrapped in 1926.
The second is detailed above in the comprehensive post by John and Nigel.
The mail boats carried the passengers from the Admiralty Pier and the cargoships carried railway cargo loaded and discharged at the Continental Goods Shed on Clarence Quay. See photos below.
Clarence Quay is in the top right corner in front of the Lord Warden Hotel, with the Granville Dock in the foreground.
One of the earlier cargoships in the foreground with the Continental Goods Shed and Lord Warden Hotel in the background.
Two of the later cargoships in a similar view.