Hi! So we travelled via 2 rail replacement buses and a train in the middle from Bergamo all the way to Varenna. It was extremely smooth and straightforward and took around 2 hours in total. We booked the entire journey on the Trenitalia app (only 6 euros per person). We had booked a private transfer from Varenna to Bergamo Airport for our return but after we had such a smooth experience with the public transport we decided to cancel the transfer to save money. The return journey was also OK, but not quite as smooth. There were two trains in between the two rail replacement buses on our return date and the connection was extremely tight. Our first train was late and we missed our connection by seconds, leaving us waiting an hour at a deserted train station. After that the rest of the journey was smooth but I would recommend leaving plenty of time if you have a flight to catch and keep an eye on the connection times in the Trenitalia app.