It seems to me that whenever (well, almost...) a locomotive builder starts with a clean sheet -- so, MAJOR redesign, not modifications of earlier types -- the result is likely to be aesthetically successful: the carbody gets designed as a whole, so it is a unified composition. But this doesn't happen often. What you get is a succession of modifications, with new details added piecemeal. The aesthetic results tend to be disastrous.
Mind you, even a new design isn't always done right the first time.
Case study: EMD hoods from the 1950s to the 1960s. Everybody agrees that the GP-7 is a good design. Aesthetically it wasn't improved on by later first-generation geeps. (Decorative skirting around fuel tank removed: my puritanical, FORM FOLLOWS FUNCTION, conscience says this was a good thing, but my eye doesn't like it. Low short hood: certainly a good idea, but the long, sloping, nose on a GP-20 looks weak.) Time for a major revision, and we got the GP-30. Oops! "Skyline casing" in front of dynamic brakes a mistake. Rounded profile of sides of cab roof... just silly looking. Second try: GP-35. Clean, functional, easy-to-produce form: utilitarian, but solidly honest. No major changes for twenty years.
Case study: GE U-series and later. U-25B is (in my-- maybe biased since I am a GE fan-- opinion) the most artistically successful hood unit of all time. Sloping short hood on late units: better for operator, but I would say not so good for spectators. Rearrangement of equipment on platform in 1966 (= between the early and late U28 phases) allows for aesthetic redesign. Shorter short hood means you don't have the silly long slope of the late U25B. One definite improvement: rear end of long hood made vertical instead of slanted. I would claim that a U30B or U23B is right up there, just after the GP-7 aesthetically... though the need for cooling capacity leads to a worrisome bulge at the rear end... Later modifications are, I think, all downhill aesthetically: radiator "wingspans" on higher horsepower models maybe aesthetically a toss-up. "Step" in side of long hood (widens in front of radiator compartment to allow more room for cross-wise mounted equipment inside) on Dash-7 a definite step downwards. Enlarged, boxy, dynamic brake housing on last B30-7A(B) and C36-7 ... aesthetically disastrous!
Time for a change. All the work on the initial Dash-8 series was on the technical front: the carbody, with multiple roof levels and a cab roof that doesn't mate with the long hood, looks like something made from spare parts. But GE then DID get an industrial designer to do something about it, and the later Dash-8 carbody (B39-8E, B40-8, B32-8, C40-8) is... well, disturbingly similar to a GP-35, but also clean and unified in the way the GP-35 was.
Case Study: Wide noses. These are hard to get right. The original "Canadian cab" looked (I would say) better in its GMD version than its MLW version. I suspect that is because they hired better industrial design talent! In the U.S., GE's first try (on B39-8 test unit 809) was unattractive, but GE then put serious effort into it. Part of that effort was addressed to purely functional aspects: they (showing good sense!) talked to actual enginemen about what would be comfortable and convenient. But putting all the ideas together was matter of design: art. I think the definitive GE "W" cab (on B40-8W, C40-8W, Dash 9....) is very attractive, and the units built with it are handsome.
(But GE hasn't had occasion to call in a designer since, and -- I'll call this "Hazen's Law" -- modifications tend to be bad aesthetically. The huge enlargement of the radiator area (to accommodate the gas/gas heat exchanger) on the ES-44 makes this type less handsome, I think, than its immediate predecessors. And the photos of the "Tier 4" prototype suggest that the long hood roof has been raised over the engine compartment in a less than attractive way.