Are reservations needed?

I would say that, in general, you don't need reservation for train travel in Europe. On trains that do require reservations, you can usually get them the day before without problem.

However, there are some major exceptions.

Night trains are very popular and often sell out well in advance. Nail down those reservations as soon as you can.

Some premium trains, such as Thalys, can sell out early, particularly for "passholder fares", and you might find yourself unable to travel on the day and train you want, unless you forego your rail pass and pay a lot more.

In Germany, trains are rarely booked, but there are a lot of savings fares that require advance booking and can save $$$ over rail pass fares. That is also somewhat true in France and Italy.

During our 8 week trip through western Europe and the Czech Republic we made reservations on many trains.

None were actually needed except from Switzerland to Salzburg, Austria. Had we not made reservations we would had had to sit in 2nd class.

I have found generally not. I did book Bologna to Venice a couple of weeks ago and was glad as the train was full. It was a mid morning train, mid week. Considering it's not high season I was surprised. If you are not set on days and times you will probably be OK. If you have to get from A to B at a certain time then I'd book. Look at the places you are going and if they are really popular tourist spots and also if it is a business city like Milan. Any Milan train I have travelled on has been pretty much full.

If a packed train will ruin your travel plans then you DO need a reservation. If it will not bother you to catch a different train then don't worry about reservations.

We waited until we arrived in Rome several years ago to purchase reservations for travel to Naples. Unfortunately the trains were fully booked. For our 2nd leg from Naples to Florence there was only 1 Eurostar train available that left 1st thing in the morning..early early early. We were actually staying in Sorrento which meant we had to get up at 4:30 AM to catch the first little commuter train to Naples where we caught the ES to Florence. That was REALLY a pain because we had splurged on 2 nights at a very nice and fancy hotel in Sorrento. We missed the whole last morning including the champagne breakfast all because we waited too long to book the ticket. Live and learn...and I have learned!

I won't repeat the thoughts I agree with, just a few new thoughts. We traveled in August which is holiday month in Europe for EVERYONE. That ended up being a time I wish I had reservations. Also, we made reservations for many trains in Italy for June and ended up not getting to use them due to strikes, canceled cars, etc. and it took about 6 months with VISA and the Italian train company to get the money back. I recommend using a credit card that will support you if something happens and you don't get what you purchase.