Complex sentence: The "Leica Freedom Train" was at its height in 1938 and early 1939, delivering groups of refugees to New York every few weeks until the invasion of Poland on Sept.1,1939, when Germany closed its borders.
Split into simpler sentences: The "Leica Freedom Train" was at its height in 1938 and early 1939, delivering groups of refugees to New York every few weeks. Then, with the invasion of Poland on Sept.1,1939, Germany closed its borders.