I'm thinking to stop by in Köln before Frankfurt
Is it safe for solo traveller, especially at tourist places and train station...must be careful with pickpocket ?
Of course, you need to be cautious whether you're in Cologne, Frankfurt, Prague or in your home town. Always keep an eye on your stuff, because the bottom line is if you're not from a European country you'll struggle badly to get home or access money.
Should be ok, but always be careful in train station or crowded place if you been in Latin America is gonna be easy for you.