A hospital train (or ambulance train) is exactly what its name suggests - a train that acts as a mobile hospital. There have been many variations of hospital trains since they first appeared during the Crimean War in the 1850s.
Earlier hospital trains were relatively basic, consisting of little more than transport for injured soldiers with only minimal first aid and nursing facilities onboard and therefore acted more like an ambulance. In contrast, later trains have been known to include fully equipped operating theatres and wards for recovery, most, however, fall somewhere between the two.
Ambulance and hospital trains have received a fair amount of attention from modellers over the years and there has even been some ready to run examples produced portraying ambulance trains from WWI.