I sometimes wonder whether the Chinese coming in to work on the railways and so forth mightn't have had some contribution to the banjo.

There are a couple of instruments which have a degree of resemblance to the banjo - see this page for more details, including pictures.

One is the Sanxian:
It actually originated in China and evolved into what it is today during the 13th century, which is when it became known as the sanxian. It uses a long fretless fingerboard. It has a small body and is covered on both sides with snake skin. It is played with a pick, placed on the lap and has three strings. this instrument is also known as the samsien in Japan. That sounds a lot like a description of an early banjo, though the look and tye sound are pretty different.

And the other is the Ruan or Moon Guitar, which, with it's circular body, looks quite like a banjo.

I get this vision of two guys on the railroad, one from China, one from Africa, swopping ideas for this great new musical instrument.