On the subject of near misses…
Presumably the use of the crew cab as an escape pod was not actually high on the list of design goals for the 4000 and 4100 class locomotives, and thankfully the locomotives involved in the recent derailment at Ambrose were unmanned.
The separation of the cab from the rest of the loco actually occurred due to the failure of the cabs resilient mounts under the lateral loads of the crash. As you’d expect resilient mounts are good for reducing structure-borne noise and vibration entering the crew cab but unfortunately not so good at ensuring a cab remains attached to a locomotive during a derailment.
But what about the structural design standard? As it happens the QR structural standard STD/0057/TEC (1) contained no quantitative requirements for resistance to lateral forces experienced in rollovers (2)(3). Although to be fair, if you assume that cabs are hard mounted in traditional fashion as the authors of the standard probably did, and that collision posts would form part of the cab front wall structure, then the omission is not necessarily a critical one.
But combine this omitted load in the structural standard with a design solution in the form of a resiliently mounted cab and you now have the perfect psychological framing effect to ensure that the vulnerability of a resiliently mounted cab to transverse loading is ignored.
Framing is a term used to indicate that the way in which information is presented can strongly shape the way in which we perceive the information. In this case a design standard strongly frames and constrains the discourse on crash-worthiness to a simple consideration of the specific load cases in the standard.
This near miss neatly illustrates how the strong framing effect of standards can introduce ontological risk into a design if in turn those standards are incomplete.
Conversely it also illustrates that the designers didn’t recognise that their mounting arrangement fell outside the experience upon which the design standard was based, and the risk that their design introduced.
1. Issued in Jan 1999. Note that subsequent releases of the standard also failed to address meaningfully the requirements for rollover lateral loads.
2. To provide a broader context to the issue, overseas rail regulators such as the US FRA and UK RSSB have investigated roll over protection and identified the need to specify both top and side load cases. So to does the Australian RISSB rail standards organisation in its structural design standard AS 7520.
3. Applying the traditional systems engineering rule that an unbounded and subjective statement that cannot be verified is not a requirement.