I have heard it is unsafe to take a train in Poland - is this true?
I have often travelled by train and never had a problem, Poland is a much safer country than Canada. If you compare the crime levels of both, both serious and petty crime you are from a nation that is probably close to 10 times more dangerous. I know because I have experienced Toronto first hand. Just keep an eye on your luggage etc and you will be fine.
What should I NOT carry around with me?
Passport, large amounts of cash, just as you would behave at home
How do Polish people view Canadians as?
Im Irish but live in Poland. From what I can make out the Poles are indifferent to other nations, but dislike loud Americans. As for Canadians Im sure you will be treated with the same respect as you would be anywhere :)
What kind of ID is considered "valid" in places such as nightclubs?
You are too young to be in nightclubs, Im telling your mother