Will my backpack fit on trains like the Frecciarossa or TGV?

Yes. A 40L bag will easily fit in the overhead rack above your seat. If the train is completely full, you can tuck it between your feet or in the luggage stack at the end of the carriage.

How do I handle souvenirs if I only have a small bag?

If you buy something, you must mail it home or ship it ahead. Alternatively, pack one empty, foldable nylon tote bag inside your main pack to carry extra items home.

How to master light packing for European train travel

Limit yourself to one 40-liter backpack that fits under your seat or in an overhead rack. Stick to a 'capsule' wardrobe of neutral colors that can be layered, and commit to doing laundry once a week.

Select the right bag. Choose a 30L to 40L backpack with no wheels. European trains have narrow aisles and limited luggage racks; a bag you can carry on your back allows you to navigate crowded stations and train boarding steps without needing a hand free for a suitcase.
Use packing cubes. Group items into three medium-sized packing cubes: one for tops, one for bottoms/layers, and one for undergarments/socks. This keeps your bag compressed and makes it easy to find items without dumping your bag on a train seat.
Limit footwear to two pairs. Wear your bulkiest pair of comfortable walking shoes on the train. Pack only one additional pair of lightweight shoes—either a flat dress shoe or a secondary sneaker—in your bag. Never bring more than two pairs total.
Follow the 5-4-3-2-1 rule. For a trip of any length, pack 5 tops, 4 pairs of underwear, 3 pairs of socks, 2 bottoms, and 1 light jacket. Choose fabrics like Merino wool or synthetic blends that dry quickly if you hand-wash them in a sink.

Will my backpack fit on trains like the Frecciarossa or TGV?: Yes. A 40L bag will easily fit in the overhead rack above your seat. If the train is completely full, you can tuck it between your feet or in the luggage stack at the end of the carriage.
How do I handle souvenirs if I only have a small bag?: If you buy something, you must mail it home or ship it ahead. Alternatively, pack one empty, foldable nylon tote bag inside your main pack to carry extra items home.