In mid September, Can we go from Venice through the Alps to Colmar in a day?

The trains take between 8 and 10 hours each way, changing in Milan and Basel.

If you just want to play tag and touch a wall, and don't mind no sleep, yes, you could do it on a 24 hour day. No time to visit though....

Are you driving?

I'm assuming you don't mean "To Colmar and back in day", and that you're going by train.

Going to Colmar by train is long but feasible, quickest route is Venice - Milan - (through Brig) - Basel - Colmar with 2 changes and about 8.5 hours, but it's not the most scenic as it crosses the Alps in two long tunnels.
A more scenic route would be via the Gotthard pass: Venice - Milan - Bellinzona - Erstfeld - Basel - Colmar with 4 changes and about 11 hours. To find that route, specify "via Goeschenen" on the journey planner of SBB (Swiss railways).
SBB can sell the entire journey, but it might be cheaper to buy
- one ticket from Venice via Milan to the Italy-Switzerland border at Chiasso
- one ticket from Basel to Colmar
- a "Saver Day Pass" sold by SBB to cover all travel in Switzerland on the day of your choice.

The trouble with the fastest and most direct route “through the Alps” ( by train) is the trains go through long tunnels under the Alps, so you miss most of the scenery.

If the Gotthard Panorama Express is running in mid-September, it might work for you ( depending on the departure time from Lugano). The route would then be Venice to Milan to Lugano, board the Gotthard Panorama Express there for the journey to Fluelen via Gotthard Pass. This is the route outlined by Balso above but you do not have to change trains.

At Fluelen you board a lake boat and cruise to Luzern. (You can do this part by train if you prefer to go faster). From Luzern there are trains to Basel, where you catch the train to Colmar.

It would be a long day, But scenic and enjoyable.

The "Gotthard Panorama Express" that Lola suggests is a very good idea, it takes the same "Gotthard route" as what I suggested. Look into its times, and if they don't work, then the route I described has the same scenery (minus the boat ride).

Thanks for all the information.

Balso, special thanks for your details.

We are in Colmar for 4 days.

We may come back and spend more time on Lake Lucerne and possibly Gotthard Panoramic Express.

Saver Day Pass or 3 DaySwiss Rail Pass ?

Thank you all.

To enjoy the alps without multiple train changes and luggage, we decided to fly to Basel and take day trips into Switzerland.