It's thicker in the middle and gets thinner on the left and right.
What I normally do for those is make a short horizontal line as thick as you want in the center of your document, rasterize it, then use motion blur on it with a horizontal angle as long as you need it to be.
Make a line whatever thickness you want
Convert to Smart Object
Go to filter > blur > motion blur
Make the angle set to 0 and add as much blur as you need to make the effect work
It's not necessarily thinner at the sides, but the fadeout makes it look this way.
The simplest way would be to create the line in Illustrator first then import into photoshop to finish.
- Create a line and add a 1pt stroke
- Using the width tool - expand the centre of the line to your desired width.
- Copy, paste in to photoshop
- Gaussian blur to desired effect
- Create layer mask and erase edges for added bleed.