Trying to aggregate enough minecarts with TNT in one spot to cause the mother of all explosions -> the more entities are on screen, the lower the framerate -> somewhere between 500 and 600 minecarts (framerate way below 10 fps), they all of a sudden explode by themselves -> after the explosion, no rails left.

since rail tracks shouldn't be affected by minecart explosions, my wild guess would be that at very low framerates, the game can't properly check if a cart is on a track or not anymore.