World War One: How did 12 million letters a week reach soldiers?

http://www.bbc.co.uk/news/magazine-25934407 

'During World War One up to 12 million letters a week were delivered to soldiers, many on the front line. The wartime post was a remarkable operation, writes ex-postman and former Home Secretary Alan Johnson.For fighting soldiers it was essential to morale and the British Army knew that. It considered delivering letters to the front as important as delivering rations and ammunition.

Trains ran back and forth across Picardy under cover of darkness dropping some mail off along the route and unloading the rest at railheads where special REPS lorries took them to the "refilling points" for divisional supplies.
 Regimental post orderlies would sort the mail at the roadside and carts would be wheeled to the front line to deliver it to individual soldiers. The objective was to hand out letters from home with the evening meal. It's said that no matter how tired and hungry the soldiers were, they always read the letter before eating the food.'