It would appear so, it also goes through the Salish-Shidhe council. I haven't reached that far in the almanac, are they all at war or just everyone stay in their own country status ? If the passage from a country to another requires a SIN verification or something else, this is still good in my book. If you tell me there's no way a unique train line may pass through those different countries, then I think I could add other stuff for my PCs to do; frontier trespassing should be fun.
The Great Northern Railway and its remnants go all around the Washington area, and completely bypasses Boise, so there should be no reason why it would ever interact with the Tir. It ends in Chicago, which, of course, has terminals that reach out all points within the Eastern United States, so getting to Detroit shouldn't be a problem.
Passing from one nation to another will require a valid SIN with a valid Passport on there. The railway will pass from Seattle to the SSC, through the Sioux Nation, and into UCAS. I have a few ways to go about this.
The first way is the easiest, and is also done by airports to cut down on time in Customs (more on that later). If the players do not wish to debark from the train at any time between Seattle and Detroit, all they have to do is present a valid SIN with a valid Passport that will be electronically checked already by both the SSC and the Sioux Nation even before the players embark. If either nation does not allow them to pass, they are not issued a ticket, and cannot legally board.
But the train makes stops, stopping in Spokane, in Idaho, Montana, the Dakotas, then to Minnesota and points East, and the Players will have to do the same thing if they want to pass from one nation to another. If they are UCAS citizens and they're in SSC, they have to be approved to leave by the SSC before they can go to UCAS, and this is to check to see if there are any outstanding warrants pending extradition.
The third way is the way that doesn't require any record keeping, but usually requires a bit more cash. Bribe someone on the train, usually a conductor. He'll get you on, and, if you're in a spot, may be able to hide you from any police who may come aboard to do surprise inspections.
When they arrive at whatever destination, even if they go from Seattle to a stop in the UCAS, they're going to have to go through Customs. Even though Seattle is still in the UCAS, it's surrounded by a foreign power and is a major stop for people on the West Coast and the Far East. Any fruits or vegetables they have on them will be confiscated, and any alcoholic beverage that's not in a proper container (like it's in a special box for shipping) will either be confiscated or a special duty be paid. Anything Duty-Free will be bypassed. Anything prohibited, such as weapons, explosives, flammable liquids, and the like will need licenses, and if the players don't have the licenses, not only will that be confiscated, but they'll be arrested and charged with illegal possession, transport, and import of such material. This may include drones, cyberdecks, illegal cyberware and bioware, and magical items/fetishes.
I've ruled, in my games, that in the North American Continent, any animals from any nation except Aztlan or the California Free State, are generally okay, with the following proscriptions. Any animal that's dangerous, like a Hellhound or Barghest, has to be licensed, crated, and given the proper paperwork. Pets don't really need to have a license, and if all you got is a cat or a poodle or something, the Customs officials in most nations will overlook it to speed things along, but it's best to at least have their shot-records ready in case someone gets a little snoopy. You can tweek this as you desire, reflecting perhaps some recent outbreak of some animal disease in some nation here and there. Animals from other nations outside the continent will be quarantined unless the local State Department says otherwise, and depending on the creature and the nation it came from, this could be anywhere from 2-6 months! I'm not kidding.
Then we get into something you're probably not going to have a problem with, and that's unaccompanied children. Customs and law enforcement require unaccompanied minors to be monitored by the train/plain/bus and they have to have contact information of people who sent them and of the people they're heading to. Even in the Sixth World, losing kids is not cool.
Now, there is a terminal in Boise that does have a railway going into the Tir. If your players decide to want to go to the Tir, they better be prepared to be examined and cross-examined. In my game, which is ran in the Portland area, the Tir border patrols take their jobs seriously, if, for nothing else, because they like to harass foreigners. In that same game, there is only one passenger train going from Boise to points in the Tir, and it leaves on time at noon Mountain Time on most days. If the PCs decide to want to take a trip, naturally they have to have a valid SIN and Passport, and even if they've already bought a ticket in, say, Seattle, and they make their way to Boise for some reason, they're going to have to get that checked before they board the train/plane/bus. The train makes four stops, and at every single stop, they will be checked again by local Peace Force agents, and these guys like to ask questions. One question they would surely ask is "Why did you go from Seattle to Boise if you're heading to Portland?" The Peace Force doesn't need probable cause to detain anyone; they can pull anyone off a train/plane/bus for any reason, or no reason. Even during the trip, the conductor on the train may, once again, check their tickets, just so there are no problems on that train that he'll have to answer to the Peace Force for.
Customs in the Tir is worse than having a barium enema. The lighter you pack for the trip, the better. They will go over everything, magically.
One trick I play on my players, which I learned from watching WWII movies featuring the Gestapo, is that the Peace Force will look at a SIN (fake or not), and will simply say something to gauge the reaction of the players, like, "Your travel permit isn't in order." That always gets my players. They fall for it every time! Their documentation could be in absolutely perfect order, and the Peace Force guys will do this, triggering a fight from the panicking players! Do this to them and watch the bullets and spells fly, and try to hide your laughter.