Born into a family of bankers with interests in railways and close links to the court of King Leopold II of Belgium, Nagelmackers trained as a civil engineer. As a young man he fell in love with an older cousin. When his feelings were not reciprocated, his family encouraged him to travel to the United States of America to help him recover and also further his professional studies. He spent a total of 10 months travelling throughout America during which time he was exposed to train travel on Pullman carriages. He became convinced that there was a market for Pullman type carriages in Europe. After a proposal to George Pullman to collaborate on developing the European market was rebuffed Nagelmackers returned to Europe.
In 1870 he published a proposal to develop sleeper carriages for the European market. However the outbreak of the Franco-Prussian War delayed the granting of a concession from the Belgian government and the establishment of his first sleeper carriage service.