That's kind of an obvious question. Routes are being developed in certain directions since merging lines share the same objects. Notice how the (2)(3)(4)(5) are being developed from Brooklyn. As well as finished lines are going in one direction (A)(B)(C)(D), (F)(V).
Also creating the route in the opposite direction is possible, but it require the "flipping" of objects. Some objects can be placed the same way forwards as backwards (rail, platforms, some canopies/station beams) but others need to be rotated 180 in the code and placed accordingly to the correct positions (platform rooms, switches, etc.). This is why it is easy to code routes in one direction as noted (same the direction, same placement of objects), but slightly difficult in the opposite (basic objects are there, just in the different direction, basically starting from scratch).