Greetings:
Recent events bring to mind a flurry of axel inspections that some of us did in the 1970s' after Mississippian No. 77 broke a driver axel on a fantrip. For a time, everyone was hiring testing labs to do ultra-sonic inspections on their drive axels (or finding people who could borrow the equipment to do the job for free).
Only a scope type ultra-sonic devise will do any good AND you must have an operator who can interpret what he "sees" inside the axel. I recall one axel in which you could "see" the forging flow. These inspections can be usually be carried out without dis-assembling the locomotive. If the locomotive is dis-assembled, we have used dye-penetrant testing with good results.
At The Valley Railroad, in c.1988 one of our volunteers who worked in the testing and inspection department of a large firm brought in a scope type tester a couple of days before we were to send the drivers from No. 40 out for turning. In "shooting" the axels he found one with a one inch deep crack at the wheel hub (the typical place to crack) and a hairline crack at the hub on another axel. We hadn't found the cracks because we hadn't removed the hub liners which hid the cracks. Ultimetly, we decided to install all new axels (others were getting "small" anyway). The cracks found were confirmed by dye-chcking. New AAR axels were purchased from Standard Steel Co.
This was an unpleasant surprise (read expensive) but far preferable to a broken axel (and ultimetly less expensive).
We always inspect the axels and other major components prior to doing major work. Better to do it right once...
J.David