Repainting Modern Rolling Stock

Most guys on here just recommend light sanding, especially with plastic and then just painting on top of that.

Soak the paint in 99% Rubbing Alcohol, then rub the paint with a soft cloth before it evaporates.

Andrew

I do this whenever the the surface of the model is in good condition - no deep scratches or broken/cracked surfaces that have to be fixed first.  Removing the existing paint is not without risks - so is painting over the old - but I think fewer risks than with removing the old paint.  In most cases the existing paint can be a good foundation for the new.  You need to remove all letters graphics and lines, etc. Goo Gone removes both although I just sand them down with #320 wet-dry, then go over the whole model much as I can with #400.  Then clean the whole thing with a tack cloth and wipe it down with thinner.  I put on a coat of primer - I use Rustoleum Auto primer, gray, in rattle cans: it behaves itself and dries hard in 24 hours, and it does a superior job of covering tiny scratches, etc., from years of handling, sandpaper, etc.  I then sand the loco and paint it, usually with rattle cans - they work well enough and there is a sufficient choice of colors I almost never go to the trouble of getting out an airbrush.

If you have to remove the paint, then as falsonservice says alcohol works sometimes.  Other people soak the thing in water with lots of dishwashing detergent for a day to more.  That works sometimes, sometimes it does not.  

Purple Power degreaser

is the most effective paint stripper for plastic that I have encountered. Extremely effective over a wide range of paints and as far as I can tell, totally benign.

Just soak the part in a container full.  It can take from hours to days, depending on the paint. You can get a gallon for $5 at Walmart.

If its OEM paint why would you need to remove it?