  We introduce a concept of hidden ladders for the bilayer Ruddlesden-Popper
type compounds: While the crystal structure is bilayer, $d_{xz}$ ($d_{yz}$)
orbitals in the relevant $t_{2g}$ sector of the transition metal form a two-leg
ladder along $x$ ($y$), since the $d_{xz}$ ($d_{yz}$) electrons primarily hop
along the leg direction $x$ ($y$) on top of the rung $z$ direction. We predict
that Sr$_3$Mo$_2$O$_7$ and Sr$_3$Cr$_2$O$_7$ are candidates for the
hidden-ladder material with a right position of $E_F$, from a first-principles
band calculation. Based on the analysis of Eliashberg equation for these
systems, we propose a possible occurrence of high temperature superconductivity
in these materials arising from the interband pair scattering processes between
the wide and "incipient" narrow bands on the ladder.
