Greyscale to Spot Color & Contrast Correction

[atramentous_rex]

I have a letterhead from a client that is a flat greyscale image. It is single channel black/transparent. I need to convert the greyscale channel into a single spot color, which I have done before but can't remember exactly how.

After that I need to edit the whole image so that the darkest part (left side and parts of the train) are solid - so that the spot color is at 100% and matches other solid spot color objects in the document - while retaining the relative transparency of the image. How should I go about this without messing up the train image?