English Translation
Hurry up, or you will be late for the last train.
さもないと sounds very formal and its tone doesn’t match 急げ (instead of 急ぎなさい) as well as 遅れるぞ (instead of 遅れますよ). Here are my alternative translations:
(Formal): 急ぎなさい。さもないと最終電車に乗り遅れますよ。
(Casual): 急げ。じゃないと終電に乗り遅れるぞ。
We usually say 終電（しゅうでん）more frequently than 最終電車 even in relatively formal writings. No one says 最終電車 in conversations.