A week later, he got a jar as he stood with Charnock beside a part of the track they had laboriously underpinned.  The ballast train was coming down, filling the valley with its roar, and the beaten snow heaved among the ties as the big cars rolled by.  The rails sank beneath the wheels and then sprang up until the load on the next axle pressed them down again; the snow flaked off the side of the road-bed, which was built up with broken rock.  Festing thought the movement was too marked and waited for the locomotive, which was coupled to the back of the train.

The engine was of the ponderous, mountain type, but it ran smoothly, with steam cut off, and although the ground trembled and the rails groaned as it passed, there was no threatening disturbance.

“The bank’s holding up, and this was about the worst spot,” Charnock remarked.  “We had some trouble in bedding the king posts in the slippery stuff.”

Then Kerr gave them a nod as he went by.  “Looks pretty good, and they have a full load on the cars.”

“I think we’ll wait until the train comes back,” Festing said to Charnock.  “The engineer will open the throttle wide to pull her up the grade.”

They sat down in a hollow of the bank, for a bitter wind blew through the gorge, and after a time the roar of falling gravel echoed among the pines.  Then there was a heavy snorting and the locomotive came round a curve, rocking and belching out black smoke.  The cars banged and rattled, slowing with jarred couplings and rolling on when the driving wheels gripped.  Festing waited anxiously, because the wheels of a locomotive when driven hard strikes what is called a hammer blow.

By and by the ground began to throb; the vibration got sharper, and Festing watched the track as the engine passed.  Cinders rattled about him, there was a mist of snow, but he saw the cross-ties start and the rails spring up and down.  Then the clanging cars sped past, and when they had gone he climbed down the side of the bank.

It was now bare of snow and one could see the stones.  Two or three had fallen, and the edges of the others were a little out of line.  The unevenness was marked, and although one or two of the heads of the timbers had moved, the movement might not have caught Festing’s eye had he not known the treacherous nature of their support.  He did not think anybody else would notice that they were not quite in their proper place.

“I’m afraid we’re up against trouble, Bob,” he said.

Charnock looked unusually thoughtful.  “The engineer had to start from a dead stop and turn on full steam.  That made the jarring worse, but it wouldn’t happen with the ordinary traffic.”

“Perhaps not,” Festing agreed.  “Still, you see, the frequent repetition of a smaller shock—­”

Charnock stopped him.  “It’s those confounded posts!  If we pull them out, we’ll have to cut down to the rock to find a solid bed, and there’s a mass of stone to move.  What would the job cost?”