"Stop, stop" aw cried eawt, "ther's a mon lost his hat" :- When the story teller loses his hat he clearly expects the train to stop to allow him to retrieve it. That might have been the practice with stage coaches, and indeed, the heavier stage waggons travelled at a walking pace thus enabling passengers to jump off, retrieve fallen articles and then catch-up with the waggon. Early railway passengers were totally unprepared for speed of steam locomotives. There are reports of passengers being killed in the attempt to dismount a moving train in order to retrieve a hat.

This sheet music cover from about 1860 shows a man (bottom left) losing his hat.

"We fill our barrows right up to our chin" :- Although this picture comes from much later in the century c1880 it gives a clear idea of the weight of earth moved in a single barrow.

 The "main plank wheel" is probably the wheel at the top of the barrow runs shown in this picture of the Tring cutting

The horse pulled the barrow and the navvy steadied the barrow as it was pulled upward. If the horse did not pull steadily, the barrow might become unstable. The navvy then had to judge which way to jump in order to avoid being crushed. In many cases the navvy came bounding down the ramp followed by the barrow. It was said that each man had numerous accidents of this type.

Reference: Thompson, Matt. The Picturesque Railway: the Lithographs of John Cooke Bourne (The History Press, Stroud, 2015) p41