Hi Again Ton:
The activity you reference is a 3rd party supplied activity? If so, then I have not run the activity.
If your engine does not actually need water/coal at that time during the activity, then I would suggest you simply spot your engine at the water tank for a bit... perhaps retrieving a cup of refreshment for yourself (coffee? tea?)... then carry on with the activity. It should not have an adverse effect on the outcome of the activity.
"Simple Controls" can help an inexperienced person running MSTS trains on steep grades from point A to point D (that is, no switching) but Simple Controls is apparently not good for switching.
When handling heavy cuts of cars up on the Buckhorn Branch, just remember to keep your speed down... use your train brake air wisely, and use that engine brake too! You don't want to have a run-away back down to Zinc City!
I do hope you have fun!