In its truest sense this means that locomotives will achieve the necessary reliability "on the road" with very little maintenance effort. Good reliability is of major importance because (i) in-service failures which disrupt other services will be increasingly less tolerated, (ii) the intensive servicing and maintenance which steam received in the past (and which is still required on today's heritage locomotives) will become too costly, and (iii) spare parts will be expensive as they will tend to be special items manufacture in small quantities.
Even at the present state of the art, reliability and simplicity do go together, and the format of the Class 5AT - a 2-cylinder single expansion 4-6-0 - is about as simple as a main line locomotive can be.
Reliability is also very much a question of good detail design, and every attention will be given to this point at the detail design stage. Naturally features of proven high reliability, such as roller bearings, will be incorporated wherever possible.
The level of reliability will be such that it is expected that major overhauls will only be required at approximately 400,000 km (250,000 mile) intervals, with intermediate overhauls (dictated only by tyre wear) at some 200,000 km (125,000 mile) intervals, and major servicing (e.g. boiler washout) at a minimum of 20,000 km (12,500 miles).
It is also instructive to read Wardale's book "Red Devil and Other Tales from the Age of Steam" pages 494-496 which discuss the general topic of steam locomotive reliability (but not relating to the 5AT). The text from those pages is reproduced here.
The anticipated reliability of the 5AT is also discussed in some detail in the FAQ section of this website.