I don't know what you're saying about locomotives, but you do have a point that 3x as many trains would mean 3x as much wear on the rails and bed. But those are constructed very differently from roads, out of different materials, so I'm not sure what that means in practice; the article indicated the potential for damage is greatly increased by building roads cheaply enough to get damaged in the first place, then exacerbated by weather.

So if rail building costs dwarf labor costs dwarf maintenance costs, and do so even at frequency, then the claim stands, vs. buses where running 3x as frequently comes closer to tripling your bus-specific costs.