I realize we're talking about driving here, but I'm not certain this freight requirement is true for trains. CN has a mainline that passes through two northern Minnesota counties (between Warroad and Baudette).
Back in the 50's Canadian National had a passenger train on this route from Winnipeg to Port Arthur (Thunder Bay) and Duluth It crossed into the US at Warroad, Minnesota....back into Canada at Rainy River, Ontario....then at Fort Frances ON the train split with one section crossing back into the US again at International Falls MN and continuing onto Duluth. The Winnipeg-Thunder Bay train passing through the US lasted into the VIA Rail era.
Canadian Pacific’s Montreal to Boston train crossed into the US from Quebec to serve Richford, Vermont then back into Canada at Glenton QC and finally back into the US again at North Troy VT......all within 20 miles!
Trains from Detroit to Buffalo and New York City ran on 250 miles of New York Central (later Penn Central then Conrail) track across southern Ontario. Multiple trains a day, some with sleeping cars only. Hundreds of passengers passing thru Canada daily between US destinations and probably some not even realizing they were in Canada. Customs inspections were done but usually just a head-count (oh how simple things were pre 9-11 !!) Amtrak had a train on this route between Buffalo and Detroit that lasted ”˜till 1979.
And there was VIA Rail Canada's 'Atlantic' across Maine.......
There were different procedures in handling the "Atlantic" over the years. When it was a Canadian Pacific train there was usually just a cursory inspection of coach passengers by US Customs. Sleeping car passengers were just left alone. The trains made several stops in Maine at Vanceboro, Danforth, Mattawamkeag, Brownville Jct., Greenville and Jackman. Local passengers were handled between US stops but anyone boarding in the US and going to Canada was checked by Canada Customs when the trains reached McAdam New Brunswick or Megantic, Quebec. This was the same procedure when VIA took over and extended the Atlantic through to Halifax in Oct. 1979 and was still in effect when the Atlantic was first discontinued in Nov. 1981.
When the Atlantic was reinstated in June 1985 there was a whole new set of rules: Now US Customs/INS insisted everyone would be inspected. Even the Sleeping Car passengers were woke eastbound at 3am ET in Jackman, Maine. Westbound wasn’t as bad: 9pm ET (10pm AT) at Vanceboro.
It took a couple of years of negotiations but eventually the train was 'Sealed' across the US with a metal 'Tag' being placed on all doors except in the one car where the US Customs Inspector rode.....only checking those getting off in the US. The 'Atlantic' was discontinued in December 1994.