good town to spend a day and night in between venice and trieste on the train???

There are trains making that run in about 2 hours, so I don't think you necessarily need to spend your time along the rail route. In fact, I've never heard of any of the (apparently quite small) towns on the rail line between Venice and Trieste.

I really enjoyed both Padua and Vicenza when I was in northern Italy in 2015. They are quite close to Venice, but in the opposite direction from Trieste.

North of the Venice-Trieste rail route is the city of Udine, which I haven't managed to visit yet. Lonely Planet has nice things to say about it.

You could also go a bit beyond Trieste into Slovenia if that's not already included in your trip plan.

Verona's also not far off the trail and another great place to spend a day and a night. But if it were me, I'd probably just stay longer in Venice, one of my all-time favorite cities.

Personally, I hate doing only one night in a location. I would tack a day onto either Venice or Trieste. If Venice, you could do an easy day trip to the other islands in the lagoon, or hoof it over to Padova for the day.