I wash the tracks in hot, soapy water (ordinary dish detergent) and air dry them. I then simply soak the tracks in a small plastic cup covered by the cold bluing solution. I’ll use a wooden shish kabab stick to stir and agitate.
(The cups I use are recycled single-serve fruit cups and the wood sticks come a zillion in a package - so I simply throw all of this mess away after stuffing a couple of paper towels in to the cup to absorb the gun blue.)
The tracks will eventually stop turning color as the solution is used up. If they’re not dark enough, I’ll add more solution and soak longer.
Once they as good as I want, I’ll wash them again in hot soapy water, scrubbing with an old toothbrush. I suppose you could give them a dunk in a baking soda and water solution to work as a sort of “stop bath.” However, I’ve never experienced any issues after simply washing the gun blue solution off. It usually is done reacting anyways.
The harder the scrubbing, the grayer-blacker the tracks will look, but I’ve found gun blue generally turns most metal tracks to a rusty brown color. The chemically colored layers can be sanded with fine wet-or-dry sand paper or buffed with 4x0 steel wool to give areas a natural bare metal shine. The areas where the road wheels travel can be masked to protect them from later weathering.
Note that chemical coloring is only one step for my track finishing routine.