I will be traveling by train to Paris est and and then needing to board a train at Paris Nord to London /St. Pancras. How do I get to the subway at Paris est to the subway to Paris Nord. Please tell in detail. I also need to know how to purchase a subway ticket between Paris est and Paris Nord. all details will be helpful. I have been doing some reading and wondered if I have figured it out acurately that at Paris Nord I will be taking a RER train most likely RER B to London?