Hello Burbear and welcome to the ArtRage forums 
Yes, this is possible if you are using a pen & tablet, where the degree of pen pressure on the tablet can be used to influence the line width with certain tools, notably brushes like the Oil Brush and Watercolour Brush, etc...
For "Hard" tools like the Pastel and Pencil, etc..., by default pen pressure won't change their width (just as in real life) but you can set this up if you want it by going into 'Set Stylus Properties' in the tool 'Settings' panel for those tools.
However if you are using a mouse then unfortunately there is no way to change a line's width while you are actually drawing it.
Maker Of Replica Macoys
Techie Stuff:
ArtRage 6.1.1 ~ 15" Macbook Pro ~ macOS 10.15.7 ~ 4 Core i7 3.1GHz CPU ~ 16GB RAM ~ Wacom Intuos4 M