The first time the signal-man sees the figure at the entrance of the tunnel, he advances toward it, and the figure disappears. The signal-man then runs into the tunnel for about "five hundred yards" and holds out his lamp to illuminate the tunnel in front of him. Upon doing this,...
the signal-man sees nothing except for "the wet stains" on the walls. The signal-man thus assumes that the figure cannot be hiding in the tunnel, and we, the readers, can be confident that this is true for three reasons.
Firstly, the signal-man runs for quite some distance without seeing the figure. Secondly, when he holds up his lamp, the signal-man is able to see the tunnel stretch ahead of him and thus would likely see any figure that had managed to run ahead of him. Thirdly, there is only one way into and one way out of a tunnel, meaning that the figure could not have reasonably escaped the signal-man's notice.
After exiting the tunnel the same way as he had entered it, the signal-man looks all around his cabin and does not see the mysterious figure. He looks "all round the red light" and climbs "the iron ladder to the gallery," but there is no sign or trace of the mysterious figure. Nonetheless, it is still more likely that the figure, when it disappeared, would have disappeared into the open space around the tunnel, as opposed to the enclosed space inside the tunnel that the signal-man has already checked. For this reason, the signal-man becomes convinced that the figure is not hiding inside the tunnel.