My research so far is suggesting to me that 631 Company Royal Army Service Corps was involved in the transport of arms and ammunition down the River Thames. There was an Artillery garrison at a place called Shoeburyness much closer to the end of the estuary and I think that 631 handled the convoys of small boats which sailed from Woolwich Arsenal down to Shoeburyness. As there was a convoy of boats, one behind the other, this was probably called a "Water Train" - like a train with carriages but with boats and on the river. Their building may have been A44, which was on the edge of the Royal Woolwich Arsenal, close to the dockyard.
Thank you for explaining what this means to me. Feel a bit silly not working out what RWA meant but we live and learn
I have somebody who has kindly offered to look at my grandfathers records. I want to thank all of you that have helped me. Trying to piece my grandfather life is like a jigsaw puzzle.He died when I was 1yr old so I only have family stores of him . He did so much for this country and to make the world a better place for all of us. He served in the Royal Field Artillery the RFC and navy merchant navy, home Gard. I don't want his story's to fade away. So when I have gathered what I can, I am going to put up a web site for him.
Thank-you all so much for your help