This question is like how long is a piece of string?
First you have to decide if it is a one-way line or a two way in which case the answer is totally different.
By one-way I mean a line which in its load carrying capacity is descending all the way and the return is empty. In this case braking power determines not pulling power. The limit of any line is what causes slippage. Thus a line in the middle of the Atacama desert - where it never rains - can have far steeper gradients than one in the jungle where it rains every day. In general for logging railroads 10% is the practical maximum.
On the other hand where you have equal two-way traffic or heavier loads going uphill then you are more restricted.
Take Ecuador, for example. I stand to be corrected but I believe that our gradient of 5.5% is the steepest adhesion worked normal narrow gauge railroad in the world. But this is what could be considered a normal railroad with both passenger and freight. Believe me starting steam on 5.5% is some task as only drivers skill prevents slippage and severe damage to the loco.
I am afraid to venture an opinion on the past as I am sure that fountain of all knowledge, John Kirchner will prove me wrong if I start to do so but I do not think the G & Q destroyed any locos through slippage but like when I said none of them had blown up and was corrected I am probably wrong as my knowledge is relatively recent.
From a practical point of view as a railroad engineer I would say that 4% is enough and after that go and get yourself a rack. The gradient on the G & Q destroyed it financially and will probably do so again.
So it depends what type of string, what width of string, and what you are using the string for. There is no one answer.
Michael Grimes
