Back in the early days of large screen CRTs (19 inch color), late 80s. I was working on bringing X up on a new display board .... suddenly the whole screen sort

Back in the early days of large screen CRTs (19 inch color), late 80s. I was working on bringing X up on a new display board .... suddenly the whole screen sort of twisted and shifted, and the went OK again .... outside my window a really long truck with recently replaced BART (electric train) rails was going by .... essentially a large bar magnet ....