Paris (Austerlitz), Berlin Hbf(about 5 euro), Zurich HB, Munich Hbf
I didn't see anything in Vienna 2 years ago(looked for at 3 stations)...
in Bratislava there is but did not work last year
look here: www.mcclean.ch/html/ it's web site of company which has shower/toilet facilities in mentioned on its web site places. good but expensive. 8 euro for shower. in Switzerland costs 12CHF.
also i have used one time in Tarbes (south of France; has direct night connection with Paris) for 1,5 euro only!