This process done one time only, was an optical process with encoding /
decoding during the entire record / playback process, which was called
Fantasound.  The last time, the 6 track negative was transferred was in
1955 using equalised phone lines in Burbank.As I understand, this was a
transfer to a magnetic analog medium, which is good for long term storage
(print through and Signal to noise ratios are decent even in present day
standards.

As outlined in Howard Tremains' Audio Encyclopedia, the Proper delivery of
Fantasound, was possible only with the aid of a special truck (which pulled
up to the theater) with the proper playback equipment during the initial
release of Fantasia.  With the advent of 6 track magnetic striping on
release prints, the subsequent releases took place.  This was a true leap
in technology, of which only Fantasia was the first use of Companding of
the signal, with it's resultant increase in Signal to noise ratio.

Roy Cherryhomes