A small cluster of flags and windswept tents sits near railroad tracks east of Toronto, where protesters are preventing trains from passing through a key commercial corridor, threatening Canada’s already fragile economy. Activists from the Mohawk community here say their encampment was set up to show support for a group of indigenous leaders on the other side of the country, in British Columbia, who are trying to stop construction of a natural-gas pipeline. The makeshift blockade, which…