  We consider the generalised Krein-Feller operator $\Delta_{\nu, \mu} $ with
respect to compactly supported Borel probability measures $\mu$ and $\nu$ with
the natural restrictions that $\mu$ is atomless, the
supp$(\nu)\subseteq$supp$(\mu)$ and the atoms of $\nu $ are embedded in the
supp$(\mu)$. We show that the solutions of the eigenvalue problem for
$\Delta_{\nu, \mu} $ can be transferred to the corresponding problem for the
classical Krein-Feller operator $\Delta_{\nu \circ F_{\mu}^{-1}, \Lambda}$ with
respect to the Lebesgue measure $\Lambda$ via an isometric isomorphism
determined by the distribution function $F_\mu$ of $\mu$. In this way, we
obtain a new characterisation of the upper spectral dimension and consolidate
many known results on the spectral asymptotics of Krein-Feller operators. We
also recover known properties of and connections to generalised gap diffusions
associated to these operators.
