So the question becomes, “why paint up a fancy new locomotive and leave the wheels and coupler all rusty..?”   Very simply put, the wheels and couplers take a horrendous beating.   Hairline cracks occasionally form, and if painted are difficult to detect.   Broken couplers and chipped wheels cause nasty derailments.   Rail managers feel that a little rust isn’t going to hurt anything, and maintenance forces can easily stay ahead of the problems.  Submitted by Gary Ostlund.