What is explanation of stanzas of poem night mail?

The Night Mail was a train. The Word mail itself means that it carries Letters and parcels in Scotland.Here is its line wise meaning Stanza 1

(1. indicates meaning of Line 1)

1. The Train was crossing the border( A boundary between two states/cities/countries etc.)

2. It was bringing the cheques and Posts

3. It was bringing letters for both rich and the poor

4. (In this Line they are mentioning some people to whom the letter is being sent.) The letters were for the Shop and the girl next door. (The Shop in the corner and the girl next door would be in Poet's Location.)

5. It was steadily climbing up The Beattock (The village of Beattock is located in the southern lowlands of Scotland, and lies within the administrative area of Dumfries and Galloway. It was an important stabling point for horses in the olden days with a coach house at one end of the village. It also boasts a bridge designed and built by Thomas Telford.)