Definition of "redoubtable"

This new batch was sent to Leicester shed, and the redoubtable enginemen who had made such a reputation for themselves with the ex-G.C.R. Atlantics took to the "B17s" immediately, although, of course, they required quite different driving methods; [...].

1941 September, O. S. Nock, “The Locomotives of Sir Nigel Gresley: Part V”, in Railway Magazine, page 396

Three local yobs have also joined, but they have not reckoned on a redoubtable Conductor and two local revenue protection officers who soon escort them off the train!

2022 November 2, Paul Bigland, “New trains, old trains, and splendid scenery”, in RAIL, number 969, page 57