Operating reliability of Steam Locomotives

I can understand if today's freight railroads are a bit reluctant to allow a 50-60 year old steam locomotive out on the mainline. A breakdown ties up the line and prevents the railroad from making money.

My question is how reliable were the steam locomotives of old compared to the dismals of today?

To make this a fair comparison, the question might be better stated how reliable is ONE steam locomotive compared to ONE dismal (the diesels can be lashed up so if one goes, I would guess there is a way to idle it and keep running the train,albeit with having to cut out a few cars).

I am sure part of the problem today is the lack of repair facilities for a steam locomotive.

Thanks in advance for the information shared.