Liquitex tube acrylics are formulated much like classic oil paints. This means that the colors are derived from minerals and other materials which don't fade ( re: permanent colors) either before or after application.
It also means that one tube of Titanium White is the same color as another tube...the same for the other colors.
Professional Liquitex tube acrylics and pro oil paints are also more brilliant and "alive" than lesser quality paints. The same goes for pro quality water colors too.
So, there are several significant differences between the craft, bottled acrylic paints and pro tube acrylics. Craft, bottled acrylics are not nearly as fade resistant (if at all) as pro tube acrylics. The chemistry in (meaning all that stuff that turns minerals or other stuff into "paint"...commonly called a "binder") is much more permanent and won't discolor as opposed to craft, bottled acrylics.
Because of the quality "binder" in pro tube acyrlics, I'm going to guess that they'll also be more durable if thinned and airbrushed. Because of the process to produce brilliant, consistent colors in pro tube acrylics, I'm also going to say that they'll be more opaque than craft, bottled acrylics.
One thing positive about the craft paints is that they have a LOT of different colors, and subtle shades of most of their colors. With pro tube acrylics, you'll have to mix many of your colors yourself. I'm assuming you'll also have to mix the craft paints too, but maybe not as much since there are so many colors to choose from.
The pro tube acrylics are also more expensive.
Personally, I haven't used either to paint any model trains, or anything else other than canvas, illustration board and a couple of paintings on wooden furniture. However, having used both Liquitex tube acrylics and different types of craft paints for my art and craft applications, for something I needed permanence and durability for, I'd go with the Liquitex any day.
I'm glad to see that Tom is planning on writing more about the subject.
