、。
If by any chance I miss the train, I'll go by bus.
Literal
By-chance train [at-に] miss-boarding-[conditional-たら], bus [with-で] go.
万一 is a conditional adverb meaning 'if by some chance,' 'in the unlikely event that X.' It sets a low-probability hypothetical, almost always paired with a ~たら, ~ば, or ~なら conditional clause. 乗り遅れる is the compound verb 乗る + 遅れる ('to be late for boarding') — one of several set compound verbs with 遅れる (書き遅れる, 取り遅れる).