Agoraphobia is a psychological condition where the sufferer feels overwhelmed by wide open spaces, suffering panic attacks and possible mental trauma as a result.
The condition is widespread among the Tracker population of the Amtrak Federation, the result of a thousand years of living underground in confined areas. The Trail-Blazers - the crews of the wagon-trains and Skyhawk aircraft - are drawn exclusively from the small percentage of the population that is unaffected by this malady, although they do sometimes suffer a related problem called ground-sickness. The First Family is also notably unaffected, presumably due to the existence of their overground enclave, Cloudlands, where most of their number are born and raised on the surface, allowing them to acclimatize from birth.
Although not a major problem for now, the Federation will have to deal with the issue when the time comes for its citizens to start emigrating en masse to the surface world.