its why CAN'T you say, not why CAN you say. if you are unable to say something its CAN NOT. just like French has ne (verb) pas. also you do not need to post the same question repeatedly. if they did not answer they will in due time. maybe check the box to get notified would be helpful for you. 

Lol Janet calm down. You’re being highly unpleasant both here and on other Q&A threads - I can see your comments elsewhere by looking at your profile. This is a place for everyone to learn, not for you to harass others

But "par le métro" is the one that's not correct, so 'why can you say "par le train"' was fine!

Given Janet mentions 'ne ... pas', I'll mention that modern spoken French often drops the 'ne' of 'ne ... pas'. It's helpful to know before you hear things like 'je suis pas ...' and start wondering if this means something different to 'ne ... pas' (it doesn't).