I've used both artist's acrylics and Vallejo acrylic paint as pictured both by Chris and Rob above.
The best guides for using acrylics are produced by the military modelling fraternity and they have many tutorials on You Tube.
Acrylic over gloss enamel paint does not work as there is nothing for the paint to ahdere to. However, I have in the past used acrylics on a matt enamel finish but never used enamels over acrylics. Now I solely use acrylics.
I use Vallejo Model Air for spraying locomotives and coaches and have also used thinned Vallejo Model Colour in the same manner as Rob above.
For priming brass kits I use the Halfords etch primer which gives a good key for acrylic paint. I have also used Tamiya acrylic primer but this is relatively expensive for a small rattle can.
I sometimes prime plastic kits with Tamiya acrylic primer but in most cases just brush the paint without a primer. I find Vallejo acrylic paints good as they do not leave brush marks.
For wood kits I just paint the acrylics straight on although some seal the wood with varnish first to reduce it's absorbancy.
UK railway colours are now available/becoming available as acrylic paints but there is generally no mention on what thinners to use for airbrushing. However, I have successfully thinned Railmatch acrylics with Tamiya acrylic thinners for airbrush use.
As Chris says 'it's horses for courses' and all depends on what you want to use them on.
This was airbrushed with acrylics.