Hey I see these things all the time. I was specifically inspired by a totally trashed flat car with bunks that I would drive by on my way to work. It was my prototype and I had good luck with it. What turned out to be unusual about my choice of prototypes was that it only had TWO bunks, not four like in these and most other pictures. So now I have five log cars, in various stages of bashing that all have two bunks and not four.. oh well, works for me.
I In my talking and observation it seems that the log cars are retrofitted to this use and just about proto can be bashed to make a log bunk car.
(My LHS has the white metal bunks in HO that work well. The instructions that come with them are specific for modifying a blue box flat car kit.)
I bashed my first few cars without even noticing the directions in the pack, I also modified a tanker car, no tank of course, to be a log car...
I went with 40' only. After talking with a logger here in the NW I asked him about log length and logging trucks. He said that typical non-pole loads on log trucks are under 35' so 40' flat car seemed just right. I also have a nice little HO Log truck that I trimmed down to this length and it looks just like the trucks I see on my way to work every day. That said, I do frequently see the "longer" pole loads, but these seem to require a pilot car and/or lots of flags.
Thats my ramble for today.
Dansco