Ireland for training continued and with it came the need to arrange for leave traffic. Few special trains were required at first but in July 1940, an extra steamer sailing was put on from Larne Harbour and additional trains were run from Londonderry and Belfast to connect with it. By 1941, the reverse working from Larne Harbour required a train of up to seventeen coaches. The first United States troops to land in the United Kingdom in World War II disembarked at Belfast on 26 January 1942. Although some were billeted in Belfast the majority were bound for other destinations. The NCC provided seven special trains and was thus the first railway in World War II to transport American troops in Europe. There were many more such arrivals over the following two years. The need for leave trains decreased considerably as the continued influx of Americans displaced their British counterparts and had almost ceased by mid-1943. The training programmes for British and American troops involved battle exercises for which heavy military vehicles had to be transported. The NCC converted existing rolling stock to provide 61 end-loading wagons. Having completed their training, the United States' forces began leaving Northern Ireland in the spring of 1944 in readiness for the Normandy Landings. The NCC worked hard to ensure that each train arrived in Belfast on time so that the troopships could sail with a minimum of delay. Once the invasion of Europe was underway the number of troops in Northern Ireland decreased rapidly and with them the special problems they had created for the NCC. Londonderry became an important naval base supporting the Battle of the Atlantic and a large traffic in personnel and materiel developed. At its peak in April 1943 some 20 500 men travelled to Londonderry. The facilities at Londonderry were inconvenient and rapidly became inadequate to cope with the buildup in naval activity; in late 1941 the Royal Navy started work on an additional base with extensive jetties at Lisahally about four miles (6 km) from Londonderry. The NCC already had a siding at this location and a new signal cabin and crossing loop were installed. Lisahally base was much used by allied navies during the Battle of the Atlantic and at the end of the war as a berth for captured German U-boats. Some movements of prisoners of war (POW) took place, mostly of captured submarine crews,