  We calculate two-pion correlation functions at finite rapidities based on a
hydrodynamical model which does not assume explicit boost invariance along the
collision axis. Extracting the HBT radii through $\chi^2$ fits in both
Cartesian and Yano-Koonin-Podgoretski\u{\i} parametrizations, we compare them
with the experimental results obtained by the PHOBOS. Based on the results, we
discuss longitudinal expansion dynamics.
