The guys on the Hambledon line get such good slow running with live steam as they are predominantly using Roundhose Locomotives with Summerlands Chuffers and SloMo's. You cannot get this level of control with live steam any other way, except perhaps for Geared Locomotives such as Peter Angus products.
What the guys have also done is to take the best of Garden Railway Tech, merge it with indoor Modern Modeling Techniques to create what is probably the best Indoor Garden Railway on the Exhibition Circuit.
The running is pretty much spot on and done in a Railway Like manner. They also have some back-up LGB Battery (I think) Locomotives for when a Live Steamer runs out of puff. Great line.
JonD