This is question for the boilersmiths! I'm displaying my ignorance here but when an engine completes its overhaul one of the jobs is to check and set the safety valves. Is this done simply by one person in the cab calling the pressure from the pressure gauge while the boiler dept chap sits on top of the boiler with a spanner adjusting the valve until it blows at the required pressure? It strikes me that this must be a most unpleasant and deafening task, especially on a boiler with 250psi!! Not for the faint hearted I imagine - and of course you have to hope that valve is a good 'un.