The most budget-friendly way to Interrail is to avoid high-speed and night trains that require mandatory seat reservations, which can cost between €10 and €30 per leg. Instead, use the "Rail Planner" app filter to search for "no reservation required" regional trains. While the journey may take longer, the cost is included entirely in your pass. Additionally, choose a Flexi Pass (e.g., 5 days within a month) rather than a Continuous Pass if you plan to stay in cities for several days at a time. To save on non-transport costs, focus your itinerary on Central and Eastern Europe (countries like Poland, Hungary, or Romania) where accommodation and food are significantly cheaper than in the UK, France, or Scandinavia. Carrying a reusable water bottle, shopping at local supermarkets (Lidl or Aldi) for "train picnics," and staying in hostels with kitchen facilities can easily cut your daily spending in half, allowing you to extend your trip or spend your budget on unique local experiences instead.