Customs - Going from Zurich to Strasbourg via train

In my experience, you go thru passport control at the Zurich airport, and it's usually pretty quick. The train station is in the airport. There is nothing as you cross the French border by train.
Safe travels.

You will have to change trains at Basel SBB station.
Trains from Basel SBB to Strasbourg usually (always?) leave from outlying "French trains" platforms: check platform number on the board when you disembark.
As you walk from the "Swiss trains" to the "French trains" platforms (a 5 to 10-minute walk, follow signs to platforms 30+), you will walk by a customs office, but it's empty 99% of the time and only there for spot checks anyway.

Switzerland is in the Schengen Area and has a FMOP agreement with the EU/EEA, so there is no immigration control moving between it and the other Schengen member states.

Switzerland is not in the EU single market nor is it in a customs union with the EU, so customs rules do apply. However Swiss borders are operated in such a way that all commercial traffic is filtered out into a separate area, so that there is very little need for
checking on public transport. There will be both plain clothes and uniformed customs officials from both countries on board the train, but it is unlikely you'll have to interact with them and if you do most likely you'll just be asked for your passport and if you have anything to declare.

nothing to it; find your train and board (often the track location is not posted until maybe 15-20 minutes before departure, as I recall). I've been through the station at Basel going to and from Strasbourg. A very busy place but a simple process.

The renovation of the French departure hall is now finished, and there is a nice bistro there if you need to kill some time....

you can watch train vlogs to see how Basel is set up with two sets of tracks

I'll be doing this same trip, landing in Zurich and heading to Strasbourg. I was thinking I might stay one night in Zurich after landing because I'll be tired (coming from Washington DC area with a 2 1/2 hour layover on the way), but haven't decided for sure. Are the trains comfortable for napping and stretching out a bit if I decide to push on through? Thank you!

kRS, the trains are fine, many folks sleep, especially leaning on the window, but set your phone alarm so you don't sleep past your stop. Safe travels!

But it is a relatively short train ride. 1 hour to Basel then a little walk to the French platforms, then a 40 minute train to Colmar. Most of the trains from Zurich airport require a change at Zurich main station, but the ones at 56 past the hour are direct to Basel. It ends at Basel, so no worries about sleeping past the station. Someone will wake you up so they can clean the train :-)

Not all trains are emptied and cleaned upon arrival. Some just continue on another service. Eg. many Bern - Basel trains become Basel - Zurich trains immediately upon arrival (and vv.), as I once found out...