A Falcon rocket is started from flat aluminium plates, which you weld into hoops. Then you weld the hoops together into a barrel.
Similarly with hyperloop, you have to weld the tubes from much shorter barrel sections anyway, there's no way you're going to have 100 m x 12 m plate sections that you could then roll into a pipe. And you couldn't really bend any of that anyway at once. So there certainly are weld seams between pylons.
A spiral weld might be interesting if a continuous sheet could be created at some steel mill. Some big pipes are created by welding a sheet spirally. Armadillo bought such tubes from Europe for their rockets. Though for hyperloop, bends in the pipe would require a really funny shape for the sheet. Both width and orientation of the sheet would have to change, and it would require very accurate welding ).
Similarly with hyperloop, you have to weld the tubes from much shorter barrel sections anyway, there's no way you're going to have 100 m x 12 m plate sections that you could then roll into a pipe. And you couldn't really bend any of that anyway at once. So there certainly are weld seams between pylons.
A spiral weld might be interesting if a continuous sheet could be created at some steel mill. Some big pipes are created by welding a sheet spirally. Armadillo bought such tubes from Europe for their rockets. Though for hyperloop, bends in the pipe would require a really funny shape for the sheet. Both width and orientation of the sheet would have to change, and it would require very accurate welding ).