What abnormal things were happening to the signalman?

In the story, the signalman reports a number of abnormal goings-on while he is at work. 

The first of these occurs about a year before the signalman meets with the narrator. In this strange incident, the signalman sees a ghost for the first time. It startled him because it called out, “Halloa! Bellow there!," and then exhibited some strange behaviour:

I never saw the face. The left arm is across the face, and the right arm is waved – violently waved.

It was standing at the entrance to the tunnel and, when the signalman approached, disappeared. Six hours later, however, a train crash occurred on the line, killing and wounding a number of people.

About six months after this, another abnormal incident occurs. This time, the ghost appeared by the red light but did not make any sound or wave its arms. Instead, it was leaning against the shaft of the light and held its hands over its face, as though in mourning. A short time later, a “beautiful young lady” died in a train compartment on the line.

About a week before the signalman’s meeting with the narrator, the signalman sees the ghost again. Like the other appearances, the ghost is again standing at the red light, waving to the signalman and ringing the bell. It calls out - “Below there! Look out! Look out!” - as if to alert the signalman to some imminent danger. The signalman tells the narrator that the ghost has appeared on numerous occasions during the week.

While the signalman knows that danger is coming, he is unable to grasp the meaning of the ghost’s appearance. He does not want to alert his superiors to the ghost’s warning, for fear that they will think he is mad, nor can he completely ignore these strange apparitions. The ghost thus torments the signalman and, tragically, he does not realise that it is warning him of his own imminent demise.