General Train Questions

I typically purchase my tickets directly from the SCNF train website, and this way they are available for me to take a screenshot and have easy access in the APP, which you said you already downloaded. My experience has been the Rome2Rio gives an idea of travel options and a map, but I don't think the prices or travel times are accurate. I use Trainline as an information resource, but would never purchase my tickets through it.
I did a cursory check of Rouen Rive Droite to Saint Malo on the SCNF-connect.com website. Prices are typically more reasonable the farther out you book. For example, one trip leaving in September at 11:00 a.m. requires one connection in Paris where you would need to switch stations, and is 70 euros. The prices for July and August are significantly higher, but those months are the height of summer travel, and the "deals" are probably gone. Not knowing your specifics, it is hard for me to give you precise information. Happy travels!

Having been to St Malo and Rouen your fastest way is take train from Rouen to Paris Gare St Lazare. Then you need to either take a taxi or the metro to Paris Gare Montparnasse where there you can get a fast 3 hour 30 minute train directly to St Malo.
What specific date are you searching for? It could be a matter of searching too far in advance before tickets are available for a route.

SNCF is the official app for French trains. I would use it when possible. Trainline and Rome2Rio are third parties. When are you traveling? Some routes are not available until 1-3 months from travel dates.

Rome2Rio is kind of like using Google flights to see plane routes. It lists all potential routes, but that doesn’t mean those routes are always for sale.

Thank you so much everyone. We are travelling at the end of September from Rouen to Saint Malo. Guess I will try messing with the SNCF app. I guess having to go back to Paris adds time. When I looked the quickest was over 4 hours. Again, thank you for your kindness.

To try to bring the fare down I would break the journey down into two parts by avoiding Paris, doing two searches.
It will be no faster, but should be cheaper.

First Rouen to Caen.

then Caen to St Malo. You should get a choice of either changing at Dol, or changing at Le Mans and Rennes.

Everything that "Jazz+Travels" said. Most trains through France go through Paris unfortunately. You also may be a little early. SNCF sometimes doesn't publish all the routes until a couple of months ahead of time.

Most trains through France go through Paris

I'd modify this to say that trains in France work on a hub-and-spoke system, much like airplanes do in the USA. Hubs include Paris, Lyon, Bordeaux and a few others. Going from one small city to another will usually require changing trains at a hub station unless the two places are located on the same spoke.

Rome2Rio is totally unreliable. It absolutely cannot be trusted. SNCF runs the trains, so it is a reliable information source--though on rare occasions you might run into a technical website glitch; that can happen with any website.

As others have pointed out, the fastest route between two regional cities in France is often through Paris. If a lot of extra miles are involved, as in this case, the ticket cost will be high because you are paying for those miles. In addition, you often end up needing to change train stations in Paris, which is an annoyance when burdened by luggage..

Traveling via Caen sounds like a good idea to me

The only thing I'd add to the guidance you've already received is to enter pretend dates to get a better idea of the schedules. For instance, I want to travel in early October from Paris to Dijon. If I use that date (a Monday) I only get a few choices, but if I enter a Monday in August, I can see that there are other times, with faster trains, available. I'll keep an eye on the SNCF site so when my preferred time opens up on the day I want, I can get my ticket. As someone else mentioned, it's probably too soon for you to see all the options you'll have.

I agree with the others that Rome2rio is a good site to get a general idea, but ordering directly from SNCF is best practice.

Listen to what Stuart (isn31c) says.

Also, yes, you will have to take a bus. The train goes only as far as Pontorson, and then you take a bus to get the last bit to St Malo.

If your trip will have you staying in Bayeux for any days, you could see if the Hotel Churchill is still running its daily van to and from Mont St Michel. That would be alot less hassle.

believe NO details from R2R, general ideas maybe, but no routes and no fares. It was once a bit more accurate but not now. Best now for its amusement value for some of the ridiculous answers it spews out.