Series Number Id: GSGS 3660 [numbers in red overprint]. Sheet number: part of sheet 27. Edition number: 2. Secret allied railway map, June 1918. At head of map: Belgium 1:100,000. In red overprint: Secret, [Allied] Railways, Ed. 1, sheet no. 1. "Ordnance Survey, June, 1918." Grey and blue base map. Overprint in red, green and purple: railways legend shows standard gauge (red), metre gauge (purple), 60 cm gauge (green); treble, double, single, under construction, etc. Does not show railways in enemy area except as shown on base map. Shows heavy concentration of railways in Ypres Salient and its hinterland. Covers area from north of Ypres to south of Bethune, and from Gravelines and west of St Omer to Armentières and the western outskirts of Lille., Location: Research Collections; Fonds: WW1 Trench Maps: France; Box number: PC21; envelope_number: 657.