Firstly the question title should be "Do" both options make sense? :) Yes both are used, though I'd tweak the sentence with version A. - We could take a train. (hypothetical, not really sure which train, what time, etc.) - We can take the train. (Still a suggestion but more certain that there is a train.) <-- This is probably what you want to say most of the time. That sounds more natural in my opinion. It's really close, and if someone is speaking and stressing a given word they might be trying to convey more meaning on top of that. It's hard to give complete meaning in an isolated sentence.

It makes sense only if there is more than one train route available. If there is only one train between those two places, then it would be "the" train.

Thanks, Keith.

no. 'Take the trian' is normal in this case = the train, not the bus, etc.

Thanks.