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The fundamental prohibition to wheel and axle welding is metallurgical. Tires and axles are typically 0.4% to 0.6% Carbon. Striking an arc causes the formation of martensite that air quenches to glass hardness and brittleness.

On a visit to the tourist RR on Kauai, I noticed bright shiny rail face on the inside of a curved rail with shavings on the ground. Looking at the wheels on the x-limestone pit loco it was apparent the flanges had been hardfaced by welding back in its previous life. The flanges now became cutting tools. Safety is not an issue due to 2-3 mph speeds. Once again the "you never get something for nothing rule" applies.

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I believe it's legal to build up flat spots on wheels by welding as a temporary repair to get a locomotive back to the shops for wheel replacement. I know I saw it done on N-S at least once.