Originally Posted by Giggleswick

Going by train from Krakow to Auschwitz is more complicated than going by minibus, and not to be recommended. As for getting between Auschwitz and Birkenau, there is a free shuttle bus that operated every ten minutes in the summer (I just checked the frequency online). The shuttle bus might be crowded in summer, though--I don't know, as I went in late September and it wasn't.