What does he mean by this?

[Jacqueline]

He said, "she was acting like a ******* shithead today <3 but i cant refuse those traintrack smiles <3 i havent seen a look like that in a while ;D" on Twitter. I understand all of it besides the traintrack smiles. I understand that he's trying to say something creative, but... It doesn't make sense. At all. Will someone please explain?
Yeah Daphne, we talk at school haha.
He meant shithead cause he thought I was ignoring him. I wasn't, I was just busy.

[Jacqueline]

Well he said acting like a fuvking shithead so that's prolly what he meant :-)

[neveragain]

I'm guessing he means a perfect smile since train tracks have perfect squares in a straight line

edit: actually traintracks have rectangles not squares but you get the picture lol