To follow up on Eugene's response, another factor was the distance between water and coaling stops. The fuel and water capacities of the Garratts were never as high as those for large tendered medium to large power in North America. All of this is sort of deja vu, because I recall rather extensive discussions on the subject on the RAILROAD list in the mid 1990s.
I'm seriously planning on a visit to the Welsh Highland and Festiniog lines, because I want to ride the Festiniog again, and see and rided the WHR. The Garratts will be another plus - they're fascinating machines.
Charlie Mutschler