Re: Please then explain how "leaves on the line" stops these things working as designed
Exactly the same principle as hitting a drone head on at full tilt actually; conservation of momentum.
In one case, changing its velocity from 0 to 60mph, and the other changing its velocity from 60mph to less than 60mph - the many tons of iron has an inherent unwillingness to change velocity that must be overcome by the force in opposition. A drone weighing at most a couple of kg can do very little overcoming before it exhibits plastic deformation of the catastrophic kind. A ~1400 horsepower steam engine trying to overcome the inertia of 98 tons of train through a contact surface area of a couple of square feet at most requires the highest possible coefficient of friction, something that a paste made of mushy leaves doesn't afford.