The festival site is within easy reach by rail, once in Belgium your train travel is included in your festival ticket! Just make sure that you board the second class carriages, or you will have to pay extra. The easiest way is to get the Eurostar from the UK to Brussels, and connect from Brussels MIDI station onto a train to Leuven, about 20 minutes away. From Leuven there are free festival buses to take you to the site.
You could also fly to Brussels south, then take a train to Brussels MIDI, NORD or ZUID, then a train to Leuven, then the festival bus, but this is the more expensive and environmentally damaging route, plus there are baggage restrictions which there are not on the train.
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