Should pool table slate be transported horizontally or vertically?

It doesn't matter as long as it is properly secured. Most people must transport it laying flat, horizontally, simply because it is difficult to secure it properly otherwise. Held vertically, concern can be of chipping (slate chips easily) and it moving in any direction - in addition to chipping the slate can injure or kill someone in a vehicle. The hazards are reduced in most vehicles by laying it flat.