Re: Derail and how common?

John,
Some pretty good information so far. Your answer of how often hasn't really been answered because I doubt if there is a standard answer. For railroads like the Strasburg, with heavy traffic, a substantial track maintenance budget and a relatively short run, I would guess that they could go for years, or even decades without anything derailing on the main line.
On the C&TS, which is operating a long, poorly ballasted line with a lot of old ties, drainage problems, slumping hillsides, rock slides, etc., plus the effects of a six month winter shut down, they might suffer one or two derailments per year on the main line. I'm sure their goal is zero and they have been spending quite a bit of money on upgrading the track over the past few years.
There used to be quite a few down at the heels tourist lines running over old branch lines with bad ties that were candidates for frequent derailments, but most of them have either shut down or stopped running steam since the new FRA boiler rule went into effect.
Needless to say, a suitable diesel will generally handle bad track much better than a steamer. I say suitable, since the Loop's diesel is a four wheel, high axle load steel mill engine that really puts some stress on the track. The Huckleberry RR has a GE steel mill diesel that is nearly the same weight as the Loop's but it is an eight wheel unit that spreads the load over twice as many axles and a greater span of track.
I hope this helps you.
Michael Allen