hahaha, what kind of site is this?? It´s NOT a contrail, it´s a CHEMTRAIL and no further discussion with me!
Someone came here with an already made up mind I see. But since you wondered about the "grey line", to better understand how such shadow works I made this simulated image showing how a volumetric shadow looks like from different angles:
Light is made up of photons travelling in straight lines, unless the direction is bent by the interaction with reflective and refractive materials (or gravity if we are geeky). But you can think of light as a huge number of rays travelling from a light source, through space to finally reach your eyes (or the camera) where it becomes interpreted as light.
As you can see in my simulated version, the more parallel to the trail path you get the more visible the shadow becomes. This is because there are less
lit atmospheric particles in the path of the shadow below the trail, and if you view the shadow perpendicular to the direction of the trail you will have less
unlit particles in your field of view. Or a more accurate description, a perpendicular view has
more lit particles, because a shadow is the absence of light and light is
not the absence of shadows. A shadow is not
something, it is the effect of the lack of something.
Sounds tricky? But the principle is quite easy to understand, volumetric shadows becomes noticeable depending on how much shadow is accumulating in your field of vision, which is affected by things like your viewing angle and the angle of the light. As you can see in your photo, it was taken quite parallel to the direction of the trail, so the shadow became quite noticeable.