Thanks, RandallW! I remember those now.
The one that looks like an EMD E-Unit is the one I remember most clearly. One could argue that it's too passenger-train-centric or too focused on the mid-20th century, but it was effective, recognizable and memorable.
BR&P, I think you made a good point about not focusing too much on one specific segment such as passenger rail or freight rail or fallen flags. That might mean not using rolling stock in the logo. Perhaps it could incorporate something like a crossing sign, crossing gate, or signal. Just have to be careful not to make it look like someone else's logo... not only because of copyright/trademark concerns, but more importantly because a good logo is one that can't be confused or mistaken for someone else's logo.