Line merging ignores natural barriers

[abyrd]

On a set of trips taking the yellow line into Washington, DC the two branches are shown to cross the river at two different places and merge downtown. This could be quite confusing for the user since we know intuitively that the branches share a common trunk "before" they hit the river (see screenshot below).

Only a small difference in rendering would be necessary: the branch leaving from Huntington should cut north a little sooner to join the branch leaving from Franconia, before the river. But knowing when to do this is hard -- it might involve feeding transitive some "natural barrier" description (some river or mountain polygons).