The whistle that they heard warned them that they were getting near to this dangerous stretch of road, and in a few moments the sight of a train, sparks flying from the smokestack of the engine, gave them visual proof as well.  Then for a time they ran along parallel with the tracks.  Fires were burning along the railway at intervals of about a hundred and fifty yards, and at times, in the firelight, they could see a dark figure moving slowly.

“Heaven knows what this bugle means!” said Fred, as they drew into line with the tracks.  “But if we sound it they may make up their minds that we’re all right—­and I’m not anxious for them to get curious about us.”

So he sounded the bugle from time to time.  They aroused no curiosity.  Plainly these sentries thought there was nothing strange about the passage of a military automobile, nor, in fact, was there.  It was not likely that they would know enough of the general disposition of the German army to speculate as to what officers might be doing hereabout.

“Here we are!  We’re beginning to dip,” said Boris, after a time.  “The culvert Ivan spoke of must be at the bottom of this hill.  The road gets away from the railway again after that, and when we have passed there we ought to be all right.”

“There’s just one thing,” said Fred, with a frown.  “They must know just as well as Ivan that the Russian outposts lie not far beyond them.  Won’t they think it strange for us to be going full speed toward the Russian lines this way?”

“No.  I think that’s easily accounted for, Fred. There is a crossroad less than half a mile beyond that culvert.  They will suppose that we mean to take the turn.  Ivan would have thought of that, I’m sure, if there had been any danger that they would not expect us to be traveling on this road.”

“I guess you’re right, Boris.  It sounds reasonable.  And anyway, if there is a chance, we’ve got to take it.  I’m certainly not going to hesitate just for that after we’ve come as far as this.  We’ll soon know because, as you say, once we’re past that culvert, we’ll be safe.  That’s the crucial spot.”

The grade grew sharper as they descended, and the pace of the car increased.  Now, at the bottom, stretching across the white road, they could see a heavy shadow and above on what was unquestionably the railway, half a dozen lights.

“They’ve got more than a sentry there.  It seems to be a regular post,” said Fred, a little nervous, as they approached.  “I’d like to slow down here—­we’re taking this hill pretty fast.”

“Yes,” agreed Boris, who was driving.  “But it’s not just the time to slow down, is it?”

“Hardly.  We’ve got to shoot under there so fast that they won’t have a chance to find out too much about us.  The headlight will help us, too.  It ought to dazzle them so that they won’t be able to see into the car at all.  As soon as we’re close to them, I’m going to sound the bugle pretty steadily.”