It's cliche, but still waters run deep. If you are crossing a stream and there is a spot were it is not moving, it is usually a hole. Also, even if it is not a hole, where the water is not moving or is moving slower than the rest is where all the silt and slippery stuff settles.
RR tracks, I aviod them. When I can't I walk the bike over. Never step on top of the tracks, it is slick steel and very slippery (like ice) to most boots.
Logs, loft the front wheel, stay on the pegs, give it some gas, pick self and bike up from other side of log and keep riding.