Captain Train - What's Up?
Man in Seat 61 and PHeymont recommend it, but on my first attempt at using the site, it fails miserably. States that there are only four trains daily from Warszawa Centralna to Kraków Glówny and that the quickest takes 6:40 hours, average being 7:31, listing connections with lengthy layovers. That's scary!
Rozklad-pkp.pl lists plenty of daily trains taking as little as 2:20h, no layovers, just a 1-minute halt in Warszawa Zachodnia.