I don't know how much research has been carried out in relation to WW2 ambulance trains, but a certain amount of work has been undertaken on UK trains, which were formed around the time of D-Day to convey British and Allied casualties to the network of military hospitals that had been created. Retired railwaymen recall one or two wartime ambulance trains working over the line to Overton-on-Dee (near Wrexham) before and after the D-Day landings on 6th June 1944 - the earlier trips being useful test runs for the "real thing", once the cross-Channel invasion force had landed in Europe.
Former Great Western Railway driver, Jack Wilkinson, recalled taking two light engines to Whitchurch, where an ambulance train had arrived from the Southern Railway. The GWR locomotives then worked the special on to Overton-on-Dee, where the casualties were unloaded and taken to Penley and Halston Hall hospitals by road transport (he also recalled a similar emergency hospital at Oatley Hall, just outside Ellesmere).
Ken Southern, another former GWR driver, remembered "firing" on one such special working in the period before D-Day, when a long train of passenger vehicles, which he assumed was filled with wounded soldiers, turned up at Whitchurch. On reaching Overton-on-Dee the train crew asked an American military policeman or "snow drop" (so-called because of their white helmets) what had happened to "that lot behind"; the MP laughed and said "wait till you see them". At that juncture the passengers started to disembark, and within minutes the lengthy platforms at Overton-on-Dee station were thronged with a seething mass of highly-pregnant young women!