The Train

In 1944, masterpieces of modern art stolen by the Wehrmacht are being shipped to Germany; the officer in charge of the operation, Colonel Franz von Waldheim (Paul Scofield), is determined to take the paintings to Germany, no matter the cost. After the works chosen by Waldheim are removed from the Jeu de Paume Museum, curator Mademoiselle Villard (Suzanne Flon) seeks help from the French Resistance. Given the imminent liberation of Paris by the Allies, they need only delay the train for a few days, but it is a dangerous operation and must be done in a way that does not risk damaging the priceless cargo.
Although Resistance cell leader and SNCF area inspector Paul Labiche (Burt Lancaster) initially rejects the plan, telling Mlle. Villard and senior Resistance leader Spinet (Paul Bonifas), "I won't waste lives on paintings," he has a change of heart after a cantankerous elderly engineer, Papa Boule (Michel Simon), is executed for trying to sabotage the train on his own. After that sacrifice, Labiche joins his Resistance teammates Didont (Albert RÃ©my) and Pesquet (Charles Millot), who have been organizing their own plan to stop the train with the help of other SNCF Resistance members. They devise an elaborate ruse to reroute the train, temporarily changing railway station signage to make it appear to the German escort as if they are heading to Germany when they have actually turned back toward Paris. They then arrange a double collision in the small town of Rive-Reine that will block the train without risking the cargo. Labiche, although shot in the leg, escapes on foot with the help of the widowed owner of a Rive-Reine hotel, Christine (Jeanne Moreau), while other Resistance members involved in the plot are executed.
The night after the collision, Labiche and Didont meet Spinet again, along with young Robert (the nephew of Jacques, the executed Rive-Reine station master) and plan to paint the tops of three boxcars white to warn off Allied aircraft from bombing the art train. Robert recruits railroad workers and friends of his Uncle Jacques from nearby Montmirail, but the marking attempt is discovered, and Robert and Didont are both killed.
Now working alone, Labiche continues to delay the train after the tracks are cleared, to the mounting rage of von Waldheim. Finally, Labiche manages to derail the train without endangering civilian hostages that the colonel has placed on the engine to prevent it being blown up. Von Waldheim flags down a retreating army convoy and learns that a French armored division is not far behind. The colonel orders the train unloaded and attempts to commandeer the trucks, but the officer in charge refuses to obey his orders. The train's small German contingent kills the hostages and joins the retreating convoy.
Von Waldheim remains behind with the abandoned train. Crates are strewn everywhere between the tracks and the road, labeled with the names of famous artists. Labiche appears and the colonel castigates him for having no real interest in the art he has saved:
Does it please you, Labiche? You feel a sense of excitement at just being near them? A painting means as much to you as a string of pearls to an ape. You won by sheer luck. You stopped me without knowing what you were doing or why.... The paintings are mine. They always will be. Beauty belongs to the man who can appreciate it. They will always belong to me, or a man like me. Now, this minute, you couldn't tell me why you did what you did.
In response, Labiche turns and looks at the murdered hostages. Then, without a word, he turns back to von Waldheim and shoots him. Afterwards he limps away, leaving the corpses and the art treasures where they lie.

Question: Why are the  the tops of three boxcars painted white?
Answer: To warn off Allied aircraft from bombing the art train.