Coal. Not necessarily hardest because of the physical work, but because you have to think ahead and balance the fire, damper, and so on. You can just nearly shut off the oil fire, but there is a longer lead time with coal. All types take some thinking ahead, but it varies. Hardest part with wood is making sure your fire doesn't have holes in it, especially at Sumpter, where we used long lengths.
Skip Luke
Switchman (ret): Grand Canyon Railway;
Engineer (ret): Georgetown Loop RR Inc., Sumpter Valley Ry., Monticello & Sangamon Valley;
Dispatcher(ret): Illinois Central, White Pass & Yukon, Burlington Northern Ry.
Mariner, Musician, Miner