Technically, a Canadian railroad can't operate in the US (or vice versa) although law changes in the 1990s do allow a Canadian railroad to own an American railroad (or own a controlling amount of stock in it). So for example Canadian Pacific's lines in the US are actually part of the Soo Line, a US railroad headquartered in Minneapolis. If the KCS acquisition goes through OK, that will change so CP's US lines will be under Kansas City Southern with their HQ in Kansas City. Similar for CN, you can see engines in full CN lettering except for sub-lettering for US lines like the Missabe (DMIR) or Illinois Central (IC) on the cab sides for example.