  The U.S. Particle Physics Project Prioritization Panel (P5) report encouraged
the realization of Fermilab's Proton Improvement Plan II (PIP-II) to support
future neutrino programs in the United States. PIP-II aims at enhancing the
capabilities of the Fermilab existing accelerator complex while simultaneously
providing a flexible platform for its future upgrades. The central part of
PIP-II project is the construction of a new 800 MeV H- Superconducting (SC)
Linac together with upgrades of the Booster and Main Injector synchrotrons. New
transfer lines will also be needed to deliver beam to the downstream
accelerators and facilities. In this paper we present the recent development of
the design of the transfer lines discussing the principles that guided their
design, the constraints and requirements imposed by the existing accelerator
complex and the following modifications implemented to comply with a better
understanding of the limitations and further requirements that emerged during
the development of the project.
