6024 King Edward 1

There's no defined limit. It is usually considered that four is a sensible number on most locos but I can think of those where three is more realistic and others where five and even six can be on board without getting in the way. At the end of the day it is down to the inevitable risk assessment. On the NYMR it is four but an inspector can overrule this.

 

I think that is essentially confirming what @Steve said. In that paragraph, the RAIB isn't stating an externally-mandated limit; it is saying that the Strathspey Railway had defined a limit in its own rules (presumably having gone through a robust process to do so) but then had an incident in which a causal factor was that it had broken its own rule.

That seems to me basically in line with how regulation works in this area: the regulators don't set generic limits, but they do want to see that (1) you have a robust process for managing risks and (2) having defined those risks, you follow your own procedures that thereby arise. So they won't mandate "you can only have four people on the footplate" but they will raise an eyebrow if your own rules say four, and you then have an incident in which five or more were present.

I'm not a trained risk assessor, but I suspect in British practice, if you followed a robust risk assessment process, I can't think of any steam loco where you'd reach an opinion that 5 or more was safe, certainly not while hauling a train, and in some cases you may mandate fewer. (Some of our loco risk assessments only allow 3). Equally, I suspect 5 or 6 would be more acceptable on, say, some large US locos with much roomier cabs. Cab footage I've seen of the Big Boy seems to show 4 seats, and space to stand behind that. But not many Big Boys in the UK!

Tom