When the Elric brothers and General Armstrong go into the tunnel Sloth dug, and later when the search party is shown in the same tunnel, there are rails on the bottom. How can Sloth lay rails? There are no others with him, and he doesn't seem to have any materials. Or was Pride laying the rails underground as Sloth was digging?
As far as I can recall, we never actually see Sloth himself laying the rails. With this in mind, it could technically have been anyone, be they homunculi or humans working alongside them.
Pride seems like one of the more likely options, because we see very little of him for most of the series and he would be up to that kind of task. However, someone would have had to come with him to provide light, otherwise he would not be able to see what he was doing, as seen in the fight between him and Edward, where he devours Gluttony, gaining the ability to "see" with smells. I'm pretty sure that this fight takes place after the rails are shown so we can rule him out as an option, at least him working on it solo.
Wrath would have to be running the government so he couldn't do it. Lust, Gluttony, and Envy are seen too often to be able to do it efficiently, unless they cycled out working, but they had other orders. The first Greed wouldn't do anything to help out "the old man", and I'm pretty sure the second Greed came after the rails were shown, but even if that's not the case, then that's still not something he would do.
Following this line of logic, it seem that we are down to three options:
- Sloth is getting the materials from someone/somewhere and laying the rails himself
- Pride is working with someone and laying the rails
- There is no homunculus involved and it is regular humans working under "Father's" orders.
Of the three, I think that the first is the most likely, but lacking any rock-solid proof, I can not say for certain which one it is.