Alexander Popp
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The other thing I can think of would be is there was some form of obstruction in the shape of the wither summoning blocks underneath one of the 'arms' in the shape
What difficulty did you have the gamemode set to? Unlike the ender dragon, the wither will not spawn in 'Peaceful'. In my problems with wither spawning, as mentioned in another bug report, the Wither boss did not spawn below Y=0
Curved rails cannot travel up blocks and powered rails cannot be curved.