The crews on steam locomotives were skilled and experienced and totally familiar with the routes and conditions and classes of locomotives used on those routes , and were subjected to tests every six months or so as required to make sure they are still familiar with the routes and loco's.
One could say that it was similar to a skilled person knowing his car and the road to be travelled off by heart.
Drivers or engineers as they are called are constantly assessing conditions , checking gauges, planning ahead, making adjustments to braking , throttle and reversor or cutoff , and no two locomotives are exactly the same , they all had their own character and the drivers knew it. Some crews used walkie talkies but in general the whistle and hand signs were used to communicate.
It does not matter how strong a loco is pushing or pulling, the important thing is slack in the knuckles, it was important to keep the train 'tight' for smooth operation . The longer the train was the more slack and other factors like stresses and strains became to smooth operation. On long trains the locomotives would start to move and a while later the guards van would fly away with a whack and a bang if things were not done smoothly...ask some conductors...
Triple heading did not happen often as far as I know, perhaps when there were efficiency problems ,failures or very bad weather. A big negative of steam locos was that each locomotive needed a crew and could not be MU'd or remote controlled ( yet ).
Back to back I think would only happen in emergencies when there was no way of turning locomotives, and the drivers would not be in eye contact.
I am no expert but I had a lot to do with the crews and train working because I worked on locomotives and recovery teams for many years , but I am sure other trainzers will fill in where I left things out
