the crack is open. Meaning the pillar went through traction / compression all the time in use.
the weld broke due to fatigue then the crack propagated.
I’d guess poor adjustment, possibly due to the pillar sinking, and an issue in the weld. Issues in weld can have so many causes, but supposing the welder knew their job, it’s probably a poor welding procedure, poor NDT (but then 100% radio/UT is expensive for a roller coaster), maybe lack of pre/post heating…
Just looking at the picture
I’d guess poor adjustment, possibly due to the pillar sinking, and an issue in the weld. Issues in weld can have so many causes, but supposing the welder knew their job, it’s probably a poor welding procedure, poor NDT (but then 100% radio/UT is expensive for a roller coaster), maybe lack of pre/post heating…