A brake pipe joint on a rail vehicle experiences something a plant joint never does: continuous vibration, superimposed on shock loading, across a temperature range that swings with the seasons and with the duty cycle. Material selection at the ferrule is where that environment is either accommodated or ignored.
In a double-ferrule fitting the front ferrule creates the seal against the tube and the fitting body, while the back ferrule grips the tube and absorbs vibration. They do different jobs, and they do them by deforming in a controlled way. If the ferrule material is harder than intended relative to the tube, it bites too deeply and creates a stress raiser; if it is softer, it will not hold under vibration. The specification is a relationship, not an absolute.
The general rule is that the ferrule should be of the same material as the fitting body, and the tube should be softer than the ferrule. Mixing materials across a joint - a stainless ferrule on a softer alloy tube, or the reverse - changes the deformation behaviour the fitting was designed around.
Where the joint sees repeated temperature swings, differential expansion between dissimilar materials works the joint loose over time. Keeping the body, ferrules and nut in the same material family removes that mechanism entirely - which is why same-material assemblies are specified for rolling stock rather than assembled from whatever is in stock.
Underframe environments are wet, salted in winter and rarely inspected in detail. Dissimilar metals in contact, with an electrolyte present, will corrode preferentially at the less noble material. On a brake line, that is not an aesthetic issue.
In practice, for rail brake piping we would normally specify a same-material stainless assembly, sized so the tube wall is within the range the fitting was validated for, and clamped at intervals that prevent the tube run from resonating. The fitting cannot compensate for a run that is inadequately supported - which is why clamps and fittings should be specified together, not separately.