Paintshed could only reskin a small selection of default locos and rollingstock too - steam locomotives were not part of the templates included. It was a nice bonus for the beginner, but it was really limited with what it could do.
Sourdough is right though - you can reskin content yourself without Paintshed, but it does involve a bit of work buggering around with the texture files and using a bitmap graphics program like Photoshop/Paintshop Pro/GIMP etc to recolour those textures. There are guides around online which document the process, so googling 'Reskinning in Trainz' should find what you are looking for.
