Suggest you try old topographic maps, which are on-line at www.historical.maptech.com and which will show something of the military railroads in Fort Dix.
I have printed copies of the Pemberton, Bordentown, and Lakehurst 15-minute series, dated 1948.
The Lakehurst map is the only one that shows an existing complex of narrow guage tracks--in loops within a half-mile area near its lower left which would be about 3.5 miles south of Colliers Mills.
The Bordentown quadrangle indicates an "OLD RAILROAD GRADE" that extended northwestward from that complex about two miles, then westward about 8 miles toward the main area of Fort Dix near Wrightstown. Another grade runs south from this line connecting just west of Taylor Mountain. At Cranberry Hall, it joins a line coming about 5 miles southeastward from the Wrightstown area. From this junction, a line seems to have run somewhat southward about two miles to firing ranges north of Mirror Lake, which shows in the northeast corner of the Pemberton quadrangle. There's probably more.
Hope this helps.