We clean a vocational college campus; several construction and mechanical trades being taught, so the floors get heavy wear. We do periodic work on VCT, concrete, ceramic and carpets. But then there are the stairs. Noticed that the yellow safety stripes on the stair treads were getting a grubby looking, which is not simply unsightly but something of a safety hazard - bright yellow is used to increase visibility along the edge of the tread, I stress the "bright" part.
Scrubbing by hand is ineffective, and quite time consuming. Standard floor machines are too wide and, given the narrow tread and the "no place to stand" at the same level, unsafe. Most smaller machines built for the purpose seem to be electric, not battery, so you'd have a chord snaking up or down the stairwell, a bit of a trip hazard if one is concentrating on the scrubbing.
We wound up using a circular brush on a battery powered drill. Took a bit of time (much less than by hand, though), was safe, easy to maneuver, and minimal cost. Just needed a couple extra batteries.
Here's before and after.