The first time I heard it in a railway context was when Amtrak used it to describe the rolling stock they inherited from the private companies, 1950s/60s classic American stock still in day-to-day use in the 90s and much of it probably still today.
Whilst my memory of this is related to an article on Canada where older rolling stock was being referred to as heritage, from around 1990, I would tend to think that, from both of our observations, the term is an import from across the Atlantic.
A couple of years later a colleague of mine remarked about the term being used over here with some degree of puzzlement and in the end we concluded that it was just a kinder and more PC way of defining what had become, in a British context, a problem which railway managers wished would just go away.
