I'd like to know if the Montreal–Quebec City route is worth it by car or by train.
We did it the other way around, and while it’s not exceptional, the route is definitely worth it. Just make sure to plan your route ahead of time because sometimes the GPS or road signs try to take you the most direct way.
I also agree with @Lescaribous about the real savings you’d make using public transport. Plus, I remember arriving at Quebec City’s train station, and there weren’t a ton of trains. Make sure the schedule doesn’t mess with your plans.

But if you’re staying in downtown Quebec City, you don’t really need a car—you can pick one up when leaving the city. We had a hotel a bit outside the center, so the car was handy. I found a pretty cheap parking spot right in the city center, and the savings on the hotel more than made up for it. Plus, it saved us from having to carry the picnic we’d grab at lunchtime.
I’ll go ahead and share my notes from when I was planning this route. Just a heads-up: it’s the reverse direction and pretty old!
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Day 10 (Mon 10/08/2015): Route Quebec City – Montreal (270 km – 3h)
Take Le Chemin du Roy at least as far as Trois-Rivières (Highway 138), which runs along the St. Lawrence River on the north shore. Lots of stops and visits—working water mills, old-time general stores... pure joy. Check out
http://www.lecheminduroy.com/.
If you have time, you can cross to the south shore at Trois-Rivières and head back to Montreal via Highway 132:
Once you’ve crossed to the south shore, you could head north to a few villages, like Deschaillons, but that adds about 100 km (1h30 round trip + visits).
On Highway 132 heading south, Nicolet is nice, Baie-du-Febvre and its sanctuary are worth a stop, Yamaska is an old village with some charm, while Sorel-Tracy is pretty bland (industrial areas and ports without much appeal).
You could then leave Highway 132 and follow the Richelieu River on the east side (Highway 133) for about 60 km to the town of Richelieu.
On both sides of the river, you’ll pass through charming little countryside villages, some of which are famous for their ties to the Patriots of 1837–1838.
On the east side: St-Ours, St-Denis-sur-Richelieu, St-Charles-sur-Richelieu, Mont St-Hilaire, St-Mathias-sur-Richelieu, Richelieu.
On the other side: St-Roch, St-Antoine, St-Marc, Beloeil, and Chambly.
Taking Highway 132 adds about 20 minutes and 25 km (+ visits).
Taking Highway 132 then 133 adds about 55 minutes and 45 km.
Also, if you take the south shore at Trois-Rivières, you could visit a sugar shack that’s not a tourist trap, about 30 km past Trois-Rivières at the *Gîte du Maudit Français*:
231 Haut de la Rivière - PIERREVILLE, Quebec, Canada
Theoretically, it’s not open for visits outside the half-board package, but give them a call a couple of days ahead or send an email now, and they might work something out.
=> Personal experience: We dropped by unannounced and were able to visit without any issues.
From Sorel-Tracy to Montreal on Highway 132, there’s a long, monotonous stretch along industrial areas, but it gets prettier as you pass through villages like Contrecoeur, Verchères, Varennes, and Boucherville—though it’s not as bucolic as the Richelieu River route.