It was somewhere outside Barstow when wrote:

At four or five points (generally underneath the treads,
which can be removed) the frame has snapped.

What has "snapped" ? A pre-existing weld, some steel, or some cast
iron.

I'd expect any steelwork to be weldable in situ.

Steel shouldn't snap. So either the steel failed, suggesting a design
fault, or the weld failed, suggesting either a design fault, or a
technique fault. Either is weldable, but you'll need to fix any design
faults as well. This may involve adding some welded-in buttress or
splice plate.

If it's cast iron, I'd not even try to weld it. CI is a pig to weld
at the best of times, the grade used here doesn't help, and it's an
uncertain strength afterwards. You'll need to remove it from the
stair to work on it, so on-site / sent away isn't all that different
in the end.

If it's cast iron and irreplaceable, then I'd use a steel patch
alongside it. This is a difficult thing to design, because you need to
avoid further breaks where you attach the repair splice.

Alternatively throw away the cast iron one and make new brackets from
steel. With a plasma cutter (or waterjet, for bulk production) you can
reproduce an awful lor of simple iron casting in profiled steel,
welded together as a built-up construction.

--
Smert' spamionam