If we happen to miss the express, let's catch the next train.

Literal

By-chance express-train [at-に] miss-boarding-[conditional-たら], next train [at-に] let's-board.

急行列車 ('express train') contrasts with 各駅停車 (local/all-stops train). 乗る ('to board, to ride') takes its vehicle with に — 'board onto X.' The volitional 乗ろう invites collective action.