The idea that can be conveyed from the excerpt is "The speaker used to have work, but now there isn’t any."
Literature defines an idea as a thought or a motive the speaker or author is trying to convey through his work.
In the given passage, the speaker tells that he used to build railroads, the railroads are built and the trains run on them. Nut now, he has finished building the same. This gives an idea that the speaker is now jobless.
Therefore the correct option is D.
Learn more about an idea here:
https://brainly.com/question/10904230