A storm from the distant Pacific was surmounting the coastal mountains, and the shadow cast by his machine flickered and faded as the sun was covered and uncovered by cloud fragments racing ahead of the heavy thunderheads. As his shadow crept across the rolling hills in lazy curves, Bell maneuvered carefully so as not to let it fall on the Thomas Flyer racing ahead of a dust trail on the short-line tracks.

There were three men in the Thomas. Bell was too high in the air to identify them, even with his field glasses. But the massive bulk of the figure hunched in the backseat of the open auto, and the canvas arrow that had been shifted away from the main line, coupled with poor Eustace’s attempt to sabotage his engine, told him it had to be Harry Frost.

He had spotted the dust trail ten miles after he followed the canvas arrow at the Snake Dance junction and immediately shut off the noisy Gnome. Josephine was safe on the ground thirty miles back, fuming at the delay despite an official time-out sanctioned by Preston Whiteway to give Bell the opportunity to capture Frost.

Bell turned back toward the junction and restarted the Gnome. When he saw the long yellow line that was the Josephine Special, he swooped down to the train, skimmed the roof of the hangar car, which bristled with rifle-toting detectives, turned around again, and led the train after the Thomas, rising to only five hundred feet above the locomotive.

After ten minutes he thought that they would have caught up, but the tracks were empty and the dust trail gone. A broad dry creek appeared ahead, a dip in the rolling land, as the tracks began veering alongside the foothills of the Coast Range mountains. It was bridged by a long wooden trestle.

The tall detective held his control wheel in one hand and scrutinized the trestle with his field glasses. The maze of timbers would offer excellent cover for men with rifles. And they could have hidden the Thomas in its shadows. But he saw neither the men nor their auto. Suddenly he heard two sharp explosions—louder than the roar of the Gnome. He knew they weren’t gunshots. Nor had they come from the trestle but from directly beneath him, as if from the locomotive.

The big black Atlantic slowed abruptly. Its high drive wheels ground sparks from the rails as the engineer fought to stop his long train as fast as he could. The loud reports, Bell realized, had been caused by torpedoes—detonating caps of fulminate of mercury attached to the rails with lead straps to signal trouble ahead. When a locomotive passed over them, they exploded loudly enough for the engineer and fireman to hear over the roar of the firebox and the thunder of the steam.