For steam engines, the analogy of the reverser setting your "gear" and the regulator being your "throttle" is a pretty good one. At slow speeds you want high torque, so use full reverser and vary the regulator to adjust your speed. Full regulator when stopped may result in your wheels spinning, so be patient and accelerate slowly. As your speed increases, the cylinders don't need such big gulps of steam, so back the reverser off while keeping the regulator pegged. If using the HUD, set the reverser so that your steam generation and steam usage rates match. If not, watch your speed and set the reverser for the best acceleration. Once you get up to desired cruising speed, back off the regulator and adjust the reverser to maintain a match between steam generation/usage. If you use more steam than you generate, you'll end up running out of boiler pressure the cylinders will stop driving the wheels, and you'll end up coming to a stop for a few minutes until you can build your boiler pressure back. Not fun. If you generate more steam than you use, though, you're not running as efficiently as you could be. I have not yet figured out how to managing steam usage vs. generation well without using the HUD - by the time I realize I'm using more steam than I'm generating, the boiler pressure is already on an irrecovable dive.

I assume the two brakes were there because one is engine only and the other is the whole train. Obviously asking the engine to stop 20 wagons of coal by itself is asking a bit much of it