Plattsburgh International Airport (PBG), a former Air Force base converted to civilian use in 2007, has direct flights from Florida and a handful of east-coast cities. It is the cheapest airport for travellers from Montreal looking to fly south — the "Montreal South" branding is deliberate.
Amtrak's Adirondack train runs once daily from New York City to Montreal via Plattsburgh — a slow but spectacular 10-hour journey up the Hudson Valley and along Lake Champlain. The station is in central Plattsburgh.
By car, Plattsburgh sits on Interstate 87 (the "Northway"), 100 km south of Montreal and 480 km north of New York City. The Burlington–Plattsburgh ferry runs across Lake Champlain to Vermont year-round; the crossing takes around an hour and is one of the more pleasant ways to enter the city.