Definition of "backbone"

Undoubtedly it can be said that the humble 0-6-0 has been the backbone for general service, or general utility on British railways right from their earliest days, and is likely to remain so.

1945 November and December, H. C. Casserley, “Random Reflections on British Locomotive Types—1”, in Railway Magazine, page 320

With little regular employment available in East Kent the backbone of the Kent Coast passenger traffic is therefore the commuters, the not inconsiderable numbers of people who travel each day to their work in Faversham, Sittingbourne, the Medway Towns and most of all, London.

1959 April, P. Ransome-Wallis, “The Southern in Trouble on the Kent Coast”, in Trains Illustrated, London: Ian Allan Publishing, page 212

His appearance was certainly that of a hairdresser's dummy; but in the great demoralization of the land he kept up his appearance. That's backbone. His starched collars and got-up shirt-fronts were achievements of character. He had been out nearly three years; and, later, I could not help asking him how he managed to sport such linen.

1899 February, Joseph Conrad, “The Heart of Darkness”, in Blackwood’s Edinburgh Magazine, volume CLXV, number M, New York, N.Y.: The Leonard Scott Publishing Company, […], part I, page 207