We consider the problem of communication in wireline networks, modeled as
graphs with capacity constrained noiseless links. We study $k$-unicast problems, i.e. networks with $k$ source-destination pairs, sources having independent
messages for their respective destinations. We suggest a meta-theorem to the effect that whenever the problem has a suitable network or traffic level symmetry, we have traditional routing solutions to be approximately capacity-achieving. Finally, we leverage our results with recent results in approximation algorithms for so-called polymatroidal networks to show approximate optimality of separation strategies in fairly general classes of Gaussian networks.