I was born in, and lived in number 9 Walker Drive from 1951 until I married in 1976, and continued to visit regularly until Dad died in 1997. As a boy, I remember how, at the first puff of smoke or chuff in the distance, all the lads would run, hell for leather to the road bridge on Orrell Road, climb on to the parapet and cheer as a Jinty, or 4F or other dirty black loco belched thick yellow smoke all over us (earning most of us a clout over the ear when we got home smelling of sulphur and smoke). The cutting was a nice yellow/red sandstone, and successive generations used what we called a cave, really an eroded gentle cliff face, as a gang den, where you had to be prepared to run when a ganger crew came striding along the track. I can really appreciate the potential of this site for a diesel depot, and will now be following your posts with great interest.
BTW, in one of the regular but never fulfilled efforts to reopen parts of the Liverpool railways, several years ago there was a proposal to build a station in the cutting, below the terraces on Walker Drive, where you have two sidings planned. Quite what the reason for it was, I forget, and needless to say, nothing ever came of it.