For those that haven't heard, there's a new documentary of the First World War "They Shall Not Grow Old". The film has been colorized thanks to the computer processing power of Lord of the Rings movie fame Peter Jackson. Film from those days is faster than today, so the computers added frames to films. I just watched it on BBC Two and was duly impressed. Commentary is provided via voice overs of interviews of WWI vets from the 1960s, I believe.
There is some footage of some of the trench railroads. At least a minute's worth, although I can't tell you exactly how much because the feed froze at that time.
Parts are not for those who are squeamish, as you can imagine.