My fiance and I would like to travel through Europe by rail but she is a Ukrainian National and I am an American. Do we need to purchase rail tickets separately? Is there a website where we can purchase tickets for both nationalities?
Our travel dates are the three middle weeks in September of this year and we plan to visit Poland, Italy, Austria, and time permitting, Paris via airplane.
The train systems don't care about your nationality. If you go to seat61.com you'll find links to all the national rail systems and lots of advice.
Not sure why you think nationality makes a difference with train tickets. However, it can make a huge difference with regard to a visa. As an Am citizen you automatically receive a 90 day Schengen visa but do not know what a Ukrainian might need. That needs to be determine before you commit to any travel schedule.
If your fiance needs a Schengen visa you need to start on that immediately- they will need documentation to prove she has hotels reserved, enough money to pay all expenses and a return air tickey among others. generally you will need to apply to the schengen country where you willl be spending most time