The simple pendulum formula $T = 2\pi\sqrt{l/g}$ is valid only for small angles ($\theta < 15°$) where $\sin\theta \approx \theta$. Unlike spring-mass systems, the time period depends on $g$. In an accelerating elevator going up with acceleration $a$: $g_{\text{eff}} = g + a$, so $T$ decreases. Going down: $g_{\text{eff}} = g - a$, $T$ increases. In free fall: $g_{\text{eff}} = 0$, $T \to \infty$ (no oscillation). For a pendulum in a horizontal accelerating train: $g_{\text{eff}} = \sqrt{g^2 + a^2}$ and the equilibrium shifts.