"I did not see the real significance of that wreck at once"

In 'Conrad’s Covert Plots and Transtextual Narratives' (1982), Cedric Watts suggests that scenes at the Central Station indicate a ‘covert plot’. Restoring contact with Kurtz is the official reason for Marlow’s journey, but a close (re)reading of the scenes at the Central Station suggests that Marlow is caught up in a local plot designed to delay the relief of the invalid Kurtz until it is too late, so that others may lay claim to the ivory Kurtz has collected and the threat Kurtz may pose to the career prospects of other company men is lifted.

Since even Marlow expresses uncertainty about the nature of the incident, it is understandable that most readers, certainly on a first reading, miss this aspect of the novel.