The auxiliary verb 'would' is used while expressing the conditional mood indicating the consequence of an imagined event or situation. In this context the contraction of the negative form 'would not" is used as it indicates the imagined event of missing the train. Hence Option B is the correct answer.
The modal auxiliary verb "can'' is used to indicate ability or permission. Here, the ability of missing the train is not being discussed.  Hence Option A is incorrect. 
"Will/ will not (won't)' can be used to express inevitable events or intention. Hence Option C is incorrect. 
"Do not (don't)" is used to express the negative or to ask questions. Hence Option D is incorrect.