My favorite 18xx game for six players is two games of 1846 with three players each.
I've played it 7 times now in its various iterations, and I've enjoyed it quite a bit as well.
What means "in a hex field"?
It means that the location marker of the pMC must be in a hex that has track (connected to a floated Railway Corporation.) The idea is that you can't connect a pMC to the railway if no railway has reached the hex in which the pMC is located.
I assume that this would happen very rarely.
Not all that rarely. It depends quite a bit on how aggressively the pMCs retain earnings in order to buy Capex cards. You will find that different groups of players, or even the same players in different games, can vary quite a bit in how hard they push this.
The "X" markers are doing the job of an "A" marker when placed on the first row. Right?
Right. Sometimes that job is simply to support higher level markers.