I would provide one small clarification or precision to the way you framed the search of the trains.
Goods coming into Canada are all subject to a risk assessment. There's advance commercial information provided about what's on those trains, and that information is used by our targeters in the intelligence and enforcement branch of CBSA to screen through the cars to determine which ones are a risk. Before any train car does come into Canada, there is a security assessment of the goods they're carrying. That's identified in advance.
Other than that, you are correct in the sense that those goods then pass the border and travel freely to an inland rail port of entry, where they are then subject to examination and released accordingly.