Carry-on bag limits

I'll be holidaying in eastern Canada this summer, starting in Halifax, then travelling by train to Montreal, and then Toronto. After some days in Toronto, I'll take the Canadian to Vancouver. Reading the booking information, it looks like I can take on board a carry-on size luggage, a personal item (normal size backpack), and can optionally check a large suitcase. Is there any flexibility in taking on a second carry-on bag, say a shopping bag? Might pick things up to take home.

At the moment, as I see it, I can check a large luggage or just pack my carry-on and backpack really efficiently. Any other possibilities?