Sentence:  On the Bishops Stortford line, the crisis now seems to be over; the units designed for this service are the <WSD>mainstay</WSD> of the workings once again and although some of the inner suburban sets are still seen, very few L.T.S. Line units are noticeable. 
Meaning of the word marked with <WSD> tags:  Someone or something that can be depended on to make a regular contribution.