In August 1935, the King and Queen went incognito to their holiday home, Villa Haslihorn in Horw, on the shores of Lake Lucerne, Switzerland. Joséphine-Charlotte and Baudouin travelled with their parents, while the one-year-old Prince Albert was considered too young for the journey and remained in Brussels. The children returned to Belgium with their nannies on 28 August.

On 29 August 1935, the King and Queen went for a last hike in the mountains before returning home. Their chauffeur was sitting in the back of the Packard One-Twenty convertible; the King was driving and the Queen looking at a map. At approximately 9:30 am the Queen pointed out something to her husband, who looked away from the road for an instant. The car left the road, travelled down a steep slope, and collided with a pear tree. Queen Astrid had opened her door and was thrown out upon impact. Her body struck the trunk of the tree while the car hit a second tree. King Leopold was thrown from the car and only lightly injured. The car continued and stopped in a lake. The Queen died from her injuries at the scene of the accident at Küssnacht am Rigi, near Lake Lucerne, Schwyz, Switzerland.

Queen Astrid is interred in the royal vault at the Church of Our Lady of Laeken, Brussels, beside her husband, King Leopold III, and his second wife, Lilian, Princess of Réthy.

https://en.m.wikipedia.org/wiki/Astrid_of_Sweden