The iPad can be useful for quick sketching of ideas, presentations, and that sort of thing, but I would not consider it a replacement for a computer able to run full, complex graphic design applications such as Photoshop (the full version of which will not run on an iPad). You really will need a full computer to work with, though in at least some cases you might be able to carry just the iPad; that depends on your classes and what you need to do that day, though.
The MacBook Air and the new MacBook Pro, while not as light and compact as an iPad, are pretty easy to carry (though not as convenient to use on a train since they have to be opened), so I'd suggest you consider one of those to start.
Regards.