LM2596 Cin requirements
I've read in the datasheet that for Cin you need 1.5x for an
  electrolytic capacitor.


The data sheet says this on page 15: -


  The capacitor voltage rating must be at least 1.25 times greater than
  the maximum input voltage, and often a much higher voltage capacitor
  is required to satisfy the RMS current requirements.


So for the design you linked shown below: -



The 50 volt rating on the capacitor is adequate for an input voltage that might peak at 40 volts. But maybe you read about the output capacitor Cout by mistake: -


  The capacitor voltage rating for electrolytic capacitors should be at
  least 1.5 times greater than the output voltage, and often require
  much higher voltage ratings to satisfy the low ESR requirements for
  low output ripple voltage.


The general reason for both Cin and Cout sometimes requiring "much higher voltage ratings" is because with a higher voltage rating you usually get a significant improvement in current handling and lower ESR.