Does the train horn noise mean anything?

Solution 1:

Random noises are ambient sound files. You're most likely hearing a 'metal' one; the kind that like to play when you're near those giant power transmission towers. Try listening to the files in these directories:

data\sound\fx\amb\~regions\metal

data\sound\fx\amb\~region\city\creaking

Solution 2:

Are you perhaps talking about the Bad Karma sound?

It's the only sound that I would make sense of sounding like a train horn. Bad karma can only be obtained by doing "bad" things.

From the wiki:

Karma doesn't have as much of an effect on gameplay as in Fallout, Fallout 2 and Fallout 3, since reputation has a more important place.

However, it does still exist and if someone catches you performing an action that causes negative Karma against their faction you will often lose reputation with that faction; for example, stealing something from The Silver Rush where a Van Graff can see you. They may also become openly hostile despite your reputation with them not being lowered enough for you to be considered hated where they will attack you.