Well it does say treat it like a wall. The only visualization problem for ya might be the straight line that doesn't exactly follow the hexside like a wall. Just pretend it follows the nearest hexside and you'll get the wall picture.
The LOS to G3 crosses the E3/F2 wall hexside, and a wall hexside blocks same-level LOS when neither the firer nor the target is part of any of the two hexes it straddles. Similarily with G4, where the LOS crosses the E3/F3 wall hexside.
However, if you from the example removed the railway that goes between hexrows E and F (and only keep the one between D and E), you'll have clear LOS to those hexes from D2. Now, the only wall hexside crossed to G3 is the D2/E3 hexside, which is part of the firer's hex.