If your letter from London to Edinburgh was posted at Kings X at 0800, and sent on the 0830, it may have arrived in Edinburgh at 1530 and arrived at the office at 1600. The contents could be "I'm taking the 1000 train, meet me when it arrives at 1700"

If you were to then catch the 1000 train, arriving 1700, that would give the recipient time to get to Waverley in time for the arrival.