Pack around the longest walk on your itinerary. If the shoes work for that walk and for supper afterwards, you have already saved the space a second pair would take. For two nights away, put one outfit on the bed and build the rest of the bag around it.
Wear the pieces that take up space
Start with black wide-leg trousers. Sit down in them before you pack. Check the waistband seated, the pocket with your phone in it and the hem against the shoes you plan to wear. Those three things matter more on a long train ride than how the outfit looks standing still.
Pack a change close to the skin
A white ribbed tank can sit under a jumper on the journey and under an open shirt on the second day. Pack a spare top, fresh underwear and socks in one small pouch so you can change without emptying the bag.
Pack for the walk you’ll actually take.
Choose a layer that opens
A full-zip waffle layer is easy to take off in a warm carriage. Add your own rain shell if the forecast calls for it. For supper, swap the hoodie for a shirt you brought from home and keep the trousers and shoes.