A good question! All trains from SPb [Petrograd] to Mogilev passed via Tsarskoye Selo and departed from the same station - Tsarskosyel'sky Vokzal [since then renamed Vitebsky]. However, the fast train Petrograd - Kiev that departed on 10:30 PM, had comfortable coaches and was usually used for trips to Stavka, didn't stop at Tsarskoye. Its first stop after leaving Petrograd was Vyritsa. Therefore it was feasible to travel from Tsarskoye backwards to Petrograd by a commuter train, then to board the Kiev-bound train at the same Tsarskosyel'sky station.
By the way, today the situation is exactly the same: in order to get by rail from Pushkin to Mogilev, Kiev, Odessa etc. in a decent way, one has first to go to Petersburg's Vitebsky station by an elektrichka.