On the way out, The Canadian train from Toronto to Vancouver: 72,000 litres of diesel.
On the way back, WestJet 737-700 from Vancouver to Montreal (which is further): 13,000 litres of jet fuel.
The 737 carries 140 passengers.
I'm under the impression that The Canadian carries about 250 passengers in total, though I can't actually find figures.
Conclusion: per passenger mile, extreme long distance trains can end up burning more fuel than planes, due to the train carrying along bunk beds, showers, kitchens, etc., etc.
Edit: crazyscot pointed out some corrections, with most importantly the plane drinking about half what I'd assumed.