A lot of the basic onebag strategies will apply to your train trip.
- Keep your wardrobe simple/versatile
- Rolling your clothes into a packing cube can get them pretty tight/compact
- Coordinate with your friend to avoid carrying any redundant items, share whatever possible (USB brick with multiple ports for charging phones, toothpaste, shampoo, sunscreen, brushes, etc)
- Checking the forecast may help you eliminate bulky items like a rain shell or sweater perhaps
- Replace your toiletry bag with a ziploc