Originally Posted by ciurpita
... on what scale?
Pick any scale you want. If the string of boxcars forms a closed loop, you can't change it's linear momentum.
Perhaps a better example... No matter how hard to push/spin a merry-go-round, you'll never be able to change it's linear momentum (assuming is has an axisymmetric mass distribution). The point I'm trying to make is you can push on something, and even put it into motion without changing its (linear) momentum.
Why is this important? Many folks here have suggested that if a lifting wing pushes on the air, then the air's momentum must be changing at a rate equal to the lift. This is not necessarily true (it's certainly not required by Newton''s Laws).