Here are four examples of difficult but controlled climbs known to all steam fans as “A Slog” and illustrate why these spectacles draw the biggest crowds.

The first is a perfect example featuring G.W.R. King Edward I in 2010. Regulator and Cut-off have clearly been set at the bottom of the climb with the powerful locomotive left to get on with it! and although progress slows towards the summit, none of the ample additional power is ever called for.
Full original upload here:    • 6024 King Edward 1, The Royal Duchy, ...  

The second features two combined clips of G.W.R. Nunney Castle climbing Upwey Bank in 2013 and topping the summit at Bincombe Tunnel in 2011. Castles were well known as “Slogging Machines”. As a Newton Abbot Driver once said to me: “Point’er at an ‘ill and ‘er just keeps goin” and with an exhaust to shatter windows, a climbing Castle is a spectacle like no other.
Full original uploads here:    • 5029 Nunney Castle breaks the Sound B...  
and here:    • The Weymouth Seaside Express, 5029 Nu...  

The third clip features the last couple of miles of the long climb out of Newquay. The “sleeping” class 47 on the rear raises the weight of the train to 760 tons which two gallant “Black Fives” have no difficulty with over a short flat section until the approach to the infamous (old) A30 over-bridge, which over the years has been both raised and the road surface lowered to accommodate ever larger lorries. The already difficult final 1 in 40 is therefore compounded by an additional short rise to the bridge which nearly overwhelms the pair. The resulting visual spectacle which I have never seen bettered since 2013, was a filming dream come true. Note: The sometimes strange audio effects are the result of echoes from a nearby wall.
Full original upload here:    • Heroic "Black 5s" to the Rescue. Day ...  

Finally, and unusually I feature S.R. three cylinder locomotives which normally would not be convincing as “sloggers” due to more rapid and subdued exhaust beats. However, the very low speed and high power requirements results in what I consider to be “The Ultimate Slog”. Although appearing extremely tenuous, this is in fact a very controlled ascent on a very severe gradient in 2014, especially when the tightness of the curve is taken into account. 34067 Tangmere is doing the majority of the work as 34046 Braunton keeps the pressure on at the rear. Power is not the issue here but adhesion as these “light pacifics” are prone to slipping and any loss of momentum on this climb would be fatal. In the end it is careful management of power verses load which wins the day in a scene unlikely ever to be repeated as traffic on this restricted route continues to increase.
Full original upload here:    • Incredible Scenes on The Atlantic Coa...  

LOADS
The loads quoted are the "official" Network Rail loads which have been in continual use since the days of B.R. "Modernisation" and are the loads stated on sites such as Realtime Trains &etc. They are: 40.5 tons per loaded MkI & MkII coach and 55 tons per loaded tender (also part of the load on the drawbar), so for a 9 coach train we get 9 x 40.5 + 55 = 420 tons.

Although this compilation is not “new” material, all clips have been re-worked in some way and are therefore not exactly as originally published.