Don’t ever, ever expect your train to be on time. It will be a huge bonus if it is. Also don’t blame on-train staff for this. VIA has some weird policies that affect on time probability and make it a rare thing.
If the train is not busy they may allow you to claim a face-to-face group of 4 seats. This is great for sleeping.
If the train is busy and 2 of more are travelling together don’t let them know. Each person should try to get 2 seats. If you sit together at the beginning they may force you to stay together and you will have to try to sleep side by side. This is not easy. It is much easier to lay down if you have 2 seats to yourself.
Even better for sleeping is a berth. VIA sometimes had empty berths and they rent them out at a reasonable price. Ask early if you think you might need one occasionally.
Always tip the kitchen/bar staff. You will get great service either way but they might do a little extra just when you need it if you’re a good tipper.
Take every opportunity to get off the train whenever they let you, no matter how briefly. There is limited room in coach for a stroll.
Don’t bring a blanket, neck pillow and eyeshades. For a mere $10 VIA will sell you a package that includes all of the above plus earplugs. It all comes together in its own cloth bag for easy carrying or attaching to your luggage.
Speaking of luggage, if you can get everything in a carry on or backpack do so. Then you won’t have to wait for luggage at the end of the trip. The weight allowances are very generous.
You may not need it normally but I suggest you bring gravol or something similar. The train can rock, a LOT. Also pain medication like Tylenol or aspirin. They dong carry any on board.
Drink lots of liquids while travelling. It helps keep your insides moving and I also found it very dry on the train.
If your travels cover a long distance, remember to change your timepiece or phone to local time. Departures are in local time and sometimes you may have more than an hour or two off the train. Don’t get caught coming back late or the train might leave without you.