The Railroad Strike of 1950
On August 25th, 1950, Truman ordered the US Army to seize control of the railways. The motivation was simple - the railroad unions had been threatening a strike, and to prevent the shutdown that would mean, Truman instead took control in order to keep things running and to force the unions to accede. It was far from unexpected from this strongly business-minded president, especially giving circumstances surrounding the strike.
Truman had history with intervening in railway strikes, having done so before in 1948 in a far more limited capacity. This time, however, things were in far more critical condition. The Korean War was just beginning, and so fluid transport of supplies around the country was utterly crucial. A shutdown of the railway networks would cut off supplies of materials and products needed to conduct the war. Therefore, when two of the biggest unions, The Brotherhood of Railroad Trainmen and the Order of Railway Conductors, threatened to strike, it meant a serious danger for the safety of the United States. If the strike were allowed to take place, a major hole would appear in America's defenses and its war effort.
Truman's first move was to order the establishment of an emergency board that would negotiate with the unions and attempt to settle their qualms before the strike began. However, the unions rejected the conditions set outright and still seemed determined to strike by August 25th. It was then that Truman stepped in, ordering the US Army to seize control by 4 PM on August 27th. The seizure was successful, although the strike went on nonetheless.
Truman argued, perhaps truthfully, that the seizure was needed to protect America from attack and to keep its economy intact. He also used this logic when, soon after, he seized control from steel workers who had gone on strike - steel was a similarly precious resource during wartime, and so he felt it was vital to national defense that steel production continued regardless of union action or the causes for said action. In any case, the strike lasted for 21 unsuccessful months before the unions at last ceded to his terms and returned to their positions in May of 1952.
While the seizure may not have been the most legally or ethically justified, given the quite probably valid concerns of the workers, it was nonetheless successful. There is a chance that it was needed to keep America safe, as Truman states. Still, whatever the case, America kept its railways but the workers saw no improvement to the condition of their lives.