Himself hidden in the shadows he could see in the moonlight the approach of any other person.
"They're river pirates," said David to himself, "or smugglers.

They're certainly up to some mischief, or why should they object to the presence of a perfectly harmless stranger ?" Partly with cold, partly with nervousness, David shivered.
"I wish that train would come," he sighed.

And instantly?
as though in answer to his wish, from only a short distance down the track he heard the rumble and creak of approaching cars.

In a flash David planned his course of action.
The thought of spending the night in a swamp infested by alligators and smugglers had become intolerable.

He must escape, and he must escape by the train now approaching.