Don't know whether it was the world's highest claim, but it certainly was France's most onerous - in March 1976, a young lad was watching a football match with his bunch, had a few drinks too many, and on his way home with his ancient Traction avant, managed to skid and stall it on the Paris-Strasbourg main line tracks near the Marne-Rhine canal in the vicinity of Bar-le-Duc. Minutes after, a westbound freight train (there was a significant freight traffic then...) collided with the wrecked car. The locomotive derailed, destroyed the canal bridge and plunged into the canal. Several freight cars followed.
Fortunately, the train driver and his aide were unscathed.
But the locomotive (a brand new BB15000) was crushed into oblivion, as well as 20 freight cars, and obviously the merchandise - ironically, beer from the Kronenbourg brewery.
All in all, considering the loco and rolling stock write-off, the complex salvaging operation, the reconstruction works, the revenue loss to the railways, and even a complaint from the local fishing society, the damages reached 30 million francs, approx. £3m of that time.
The lad was insured with MAIF, the mutual insurance association of schoolteachers, and MAIF subscribers had to pay a "Bar-le-Duc" premium surcharge...
I am myself a MAIF subscriber and they're the best insurer one can deal with. They are a a mutual/co-op insurer rather than a purely commercial company, and it shows.