Derailment at low speed

The quick facts:

Details and piccies

This piccy shows us on our way. The car was full, all seats taken and a few standing. Lots of decorations and conversation. Behind the camera are some tables with food and drinks (tables in front of some exterior doors)

How I think it happened:

1. The facts:
1. Basic facts are above
2. Track is old (jointed, wooden sleepers, rusty rail) and maintained by volunteers
3. Track is in a rough condition, maximum speed 60 Km/h, often travelled around 40Km/h. Track visibly bumpy
4. This section of track had been passed over previously that day (in opposite direction)
2. Railcar travelling uphill and negotiating a curve to the left in a cutting. Speed estimated 20 Km/h by one passenger. I say definitely below 40 Km/h.
3. We were playing "Murder on the Orient Express" when there's a loud "Bang" and there are a few bumps and lurches.
4. People ask "What was that? What's happening?" Before I realize, somebody says "we've been derailed". I realize he's right and kick myself for being so slow.
5. Railcar drops a bit, slows down and ride gets very bumpy as we go over the sleepers (we're in the gravel). We stop.
1. People have various reactions, from surprise, shock, questions to panic: "We're going to tip over!".
6. The guard confirmed we were in the gravel, driver had a look, hopped back in and shut engines down. Guard said we were completely safe, and were not going to tip over. Panicking lady is then educated that derailment does not mean tip over and people die. Fortunately she in a shock-panic and not hysterical (sitting near me). Driver is surprised that rail isn't all mangled.
1. Driver is younger than guard, at about 20 something. This railcar is apparently his "baby", he maintains it under, out, over and in.
7. Driver got ladder out for people to exit. The more youthful ones jumped (as we were in a cutting of very convenient height)
1. After a while the birthday gal's boyfriend proposes to her, and she says "yes". Railway volunteers congratulate them upon learning of this. (Moral: good things can happen after derailments.) He had planned on doing this further up the line.
2. Once de-trained, I examined the track
8. It seems a few adjacent sleepers allowed a bit too much lateral movement. As a result, outer (RHS) rail spread, front (leading) LHS wheel dropped, creating a bit of a bump. Wheel ran just inside of rail for a few metres, then encountered fish plate (if that's what it's called). Sheared first bolt as seen in photo, which probably caused the "BANG", then rode up on the next bolt. By this stage lateral movement is not permitted, so something has to give. The front RHS wheel gives to the pressure and rides up onto the rail for a metre or so. Marks can be seen on the rail where the flange rode. Flange crosses rail and front axle of front bogie is in the gravel. Stone thrower isn't very effective against rail. Meanwhile rear bogie has reached damaged track (from front wheel), and front LHS wheel of the rear bogie drops. We are stationary. So to clarify there is now a spread section, a 'good' section with damaged fish plate, and another spread section.
9. Marks can be seen in the wood where the track spread. Since rail is a bit rusty, there are lots of fairly clear marks.
10. Some spikes in damaged section can be lifted out by hand. Obviously rail is more damaged now.
11. Volunteer rail staff debate as to cause. Theory of big stone becomes popular among passengers, local residents and some staff. (Residents loved this: an opportunity to complain about the local riffraff wrecking havoc with the railway)
12. Extra staff arrive, one with tape measure. Measures gauge, it's a bit to big back there....
13. Ask how they'll get it back. One staff member thinks the steam loco could pull it back. Don't know how a steam loco can lift though. I wonder about a big crane. Volunteer says they'll probably use a rail jack, jack up, sideways, put down. Repeat until it's back on track. Meanwhile two busses transport us back.
14. My conclusion: A few adjacent sleepers allowed a bit too much lateral movement (rail spread). If the sleepers weren't adjacent to each other, I think we would have kept going. I think this was a bad patch of about four or five sleepers, with a few good ones under the fish plate, followed by a mediocre section. I remember seeing a sleeper rotten through the centre somewhere. One passenger commented about the uneven spacing of the sleepers, but I don't think it has to be at centimetre precision! Hopefully the next track inspection would have identified this area.
1. Note: I don't know much about track work, so I could be wrong