Most efficient train routes?

How many nights will you be spending at your destination? Do you plan to rent a car?

A car is very helpful in the Dordogne. I had a very nice time there, just using trains and buses, but I had a lot of time, and I stayed in a bunch of different towns in order to facilitate some day trips I wanted to take--and those day trips were inefficent and limited in selection without a car.

What destination are you choosing as your base on Lake Como? Have you looked for lodging? The most convenient bases are around the middle of the lake. Varenna, Bellagio and Menaggio have very good ferry service. Varenna also has trains. You don't need a car at Lake Como.

Como is a lake and Dordogne is a river. Trains go to towns, not bodies of water. Do you refer to the city of Como, on the southwest end of the Lake Como? Amsterdam to Como is Amsterdam to Basel ICE, then Basel to Lugano EC, then Lugano to Como RE train. I can't find anything much less than 12 hours. Basel-Lugano is temporarily scenic until the Gotthard Base Tunnel is fully operational again.

If you want the town of Sarlat, on the Dordogne River, that looks like Amsterdam to Paris Nord Eurostar, then Paris Montparnasse to Bordeaux TGV, the Bordeaux to Sarlat TER. That is just over 10 hours. The last leg should be scenic.

I plan to stay in Sarlat (Dordogne) 4 days/3 nights. Sounds like it would make sense to rent a car while I’m there.

As for Lago di Como, I use the term Como loosely—meant Lago di Como area as I haven’t determined where to stay yet. Waiting to hear back from my cousin in Garfagnana for specific dates.

Many thanks for the excellent info.

Sarlat is a small town far from the major rail lines, so going there will take time. And it's the last bit that takes time. Amsterdam to Bordeaux is easy, 300-320 km/h high speed lines for most of the way. You'll need to change stations in Paris but it's not that hard.

Como, assuming you mean the town, is easier as the trains from Switzerland to Milan stop there.

Your options in the popular places on Lake Como may be extremely limited for June at this point. I'd recommend that you start looking now. It may come down to where you can find a room rather than where you would prefer to stay. For moving around efficiently, it's best to be in a town with frequent ferry departures. I think the buses run only once an hour, which would be quite limiting, and I understand the best views are from the water.

It’s understandable that one would think I was staying in Lake Como. I’m not actually. I’m staying a friend in the Emilio Romano region of Italy.

Emilia-Romagna is a huge region. Depending on where in Emilia-Romagna you mean that is over 200 km to 300km + from Varenna or Bellagio where many people go on Lago di Como, the middle of the lake is beautiful when I have visited there...... Bologna, Parma, Piacenza, Rimini, Ferrara are all places we can help with getting to but they are miles from each other....

To clarify, while I am visiting my friend who lives near Bologna I would like to make a day trip to La de Como. I will be coming from Amsterdam. The feedback is helping me realize that I probably wouldn’t want to do a day trip from Amsterdam, but rather with him from Bologna to Lago de Como

Well, looking at it Sarlat is not a bad journey-
Eurostar (ex Thalys) to Paris Nord at 0808, 1 hour change across Paris to Montparnasse, TGV to Bordeaux and a 1 hour change to Sarlat

or at 0908, a 90 minute change across Paris, TGV to Libourne, a 45 minute wait for the Sarlat train.

Yes it's a 10 or 11 hour journey, but a straightforward one.

(Source- SNCF Connect)

OK, so Amsterdam to the station at the southern tip of lake Como is Amsterdam to Basel, Basel to Lugano, Lugano to Como. 3 or 4 changes. But nowhere near your apparent actual destination somewhere in the large region of Emilia-Romagna. But there's your answer.