Jesse will come out with yet another girlfriend that creates a crises. Actually Walt killed the Lydia hit with the "the meth-a whatever will flow" remark. As far as the train, Mike did not want to kill "innocent men" as he said , but turn it into a small potatoes operation.
Jesse knew meth -a-whatever has water soluble because Walt told him, probably in response to his questions on the weight problem. Jesse came up with the answer, but only after Walt filled him in on some fact's.
I don't think ether Jesse or Walt would last long without the other, And their talent's dovetail perfectly.
The sum is greater than the whole of the part's so to speak.