Cracks are a royal pain to predict to be honest. The trouble with metalic cracking is when you see one the question is " is this the beginning of a crack or the end of one?"
Case in point: One locomotive I worked on was discovered to have a pinhole leak in the rear corner of the Boiler where the sidesheet makes the turn into the backhead. We pulled the fire, bled her down and had the hole inspected. It turned out , that the pinhole leak was the only sign on the outside , of a vertical crack 14 inches long. 14 inches of compromised boiler shell and all we saw was a pinhole pisser on the outside. What would have been a simple bore and weld job turned into the removal and replacement of the rear corner of the boiler's outer shell plate.
It has been my opinon that the present operation and their Press machine is generally overly optimistic with repair estimates, leaning toward what will give ,or be the least damaging to, PR.
For a crack on an engine being worked as hard as that one is, I would do an untrasound to determine just how big a " small crack" I am actually dealing with, where it begins, how deep it runs and where it goes becsause ,like the experience above, this lil crack they found may be something larger that is hidden inside the metal itself or hasnt manifested on the outside yet in its entirety. This inspection and the possible torching and replacement of boiler shell plate may take longer than their hopeful estimate but better to do so then have her yanked out of service again when the same crack appears elsewhere.