  We obtain a non-relativistic diffeomorphism invariant string action as a
special limit of the Nambu-Goto action in an FLRW background. We use this
action to study non-relativistic string dynamics in an expanding universe and
construct an analytic model describing the macroscopic properties of
non-relativistic string networks. The non-relativistic constraint equations
allow arbitrarily small string velocities and thus a `frustrated' equation of
state for non-interacting strings can be obtained without the need of a
velocity damping mechanism. Assuming that colliding string segments reconnect
by exchange of partners, non-relativistic string networks exhibit scaling
behaviour, but with enhanced energy densities due to the reduced average string
velocity. Non-relativistic string networks can be relevant in several contexts
in condensed matter physics and cosmology.
