....accessible by public transportation.
My friends have never been to Italy and I would like to have them experience a hilltop town (similar to Erice or Orvieto) for an overnight between Florence and Venice. We are traveling by train and I know that greatly limits our options but hoping there's a possibility of a bus or even a taxi/uber to get us up the hill. Pipe dream?
If it's a no-go do you have another suggestion? We will be coming from Rome and Florence and a small town would be a nice change of scenery.

You should looking for a hill town between Rome and Florence, not between Florence and Venice. Traveling north from Florence, the land flattens out pretty quickly. No hill towns unless you make a huge, time-consuming detour.

If your mental image is Orvieto, why not go to Orvieto?

Or perhaps Montepulciano. The train station is about 6-7 miles from the center of Montepulciano.

There are others between Rome and Florence you could reach by train + taxi or bus, but the travel logistics would make them unrealistic for a single night. Even the train service to/from Montepulciano might not give you enough time there, although it's mostly a town for strolling, wine tasting, enjoying the views... Very enjoyable on a nice, sunny day, not enjoyable in the rain.

I agree with Jean - look south of Florence. Consider Perugia for an overnight on the way to Florence. Train service not a problem.

Agree with what has been said. If you prefer a smaller town with train access, try Castiglione del Lago or Cortona.

It would be an add on to your Florence time, but Fiesole lies above Florence and is a very pleasant small town, especially up by the church which gives lovely views over the valley.Some fine Roman remains there too...amphitheatre etc.

Thank you for the advice, everyone. I was wondering about the terrain, Jean, but didn't know how to find out about it. Flat sums it up.
I was looking for a slow night as a break and a contrast after 9 days of Rome and Florence busyness. I should have put that in the original post. Hilltop towns have made me stop and think what it would have been like to live life fairly isolated way up above the valley floor and neighboring towns. I think my friends would dig it, too.

Bologna, which the train passes through en route to Venice, is very near the Apennines, even though it is in the vast plain of the Val Padana. There is an ancient footpath from Florence to Bologna, called the Via degli Dei. This footpath passes through the foothills of the Apennine chain. Some of the little hilltowns are very near Bologna. The only one I know personally is Brento, where we have some relatives by marriage. Brento itself is a tiny little hamlet, very attractive, but with only a few hundred inhabitants. I don't think you could find lodging or a restaurant there. However, there are many other little towns in the area. There are frequent trains from Bologna to the town of Sasso Marconi, for example, which is a small town in the foothills with a number of outlying villages. I've never been there, so I can't say much about the place.

Perhaps you don't need an actual hill to give you a feel for village life. Even nearer to Bologna, the Town of Castel San Pietro Terme is an attractive small town that you might say is on a hillock, with hills surrounding it. There is a thermal spa there, as you might guess from the "Terme" in the name. This town is also connected to Bologna by train.
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Fiesole is indeed a very nice town. It tends to be a bit overrun by tourists.

Looking for a "slow night" is a different quest than wanting a "hilltop town." As bvlenci mentions, you don't need a hill town to experience village life.

But it's not clear if this stop could be between Rome and Florence... or must it be between Florence and Venice?

I was hoping for the whole shebang, a quiet night in a hilltop town after Rome/Florence and before Venice. But I'm probably going to be modifying that plan to Rome- hilltop town-Florence-Venice. I can live with that. Thanks for the input, everyone. Perspective really helps.

If it doesn't have to be a hill town, then consider Ferrara as an overnight stop from Florence to Venice. Easy train access. Absent a hill, the rest of it may meet your criteria.

You might consider Orvieto, a hilltop town between Rome and Florence. You can arrive there by train from Rome, and you can arrive in Florence by train from Orvieto.

Orvieto is on a hilltop. In fact you need to take a funicular up the hill from the station. It's an attractive town, with a rare and beautiful Italian Gothic cathedral. It's a popular tourist destination, but most are day trippers, and by night it will be tranquil. If you go, I highly recommend the Pozzo di San Patrizio (St. Patrick's Well), which most tourists overlook.

You can also arrive in Castiglione al Lago, recommended by Jean, by train from Rome; it's a stop on the slow (but scenic) regional train from Rome to Florence. I've never been there; it's on Lago Trasimeno, and I didn't think it was on a hill. Jean can surely tell you more about it than I can.

Cortona, also mentioned by Jean, is on the same train line. It's sort of halfway up a steep hill. The town is some distance from the station, but there are probably taxis. Cortona is another popular tourist destination. There's a beautiful church, Santa Margarita, I think, a steep climb from the town center. The museum in Cortona has a very rare Etruscan chandelier, which in my opinion was the best thing about Cortona. (I was oversold on Cortona by the book, "Under a Tuscan Sun". Plus we were there on a very hot day, and the steepness of the town center was a bit much in the heat.)
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Ferrara is an excellent suggestion. It's near Bologna, and has a castle right in the center of town. It's really a small city, rather than a town, and it's not on a hill, but it has a lovely town center and excellent Emilian cuisine.There are some high-speed trains on the Florence to Venice lines that stop in Ferrara, so it would be an easy break in the journey.

Very nice suggestions, thank you very much for taking the time to respond and guide me through the towns, options and alternatives. I'll be looking in to all of them.

Yes fair enough if you know it during peak season.. we were there in March so it was quiet to the point of deserted!