The cutting edge!

 The Goods Shed and Loco facility both will have round top windows (probably a correct term for this somewhere) and cutting these in card with a conventional modelling knife can be tricky.
When in The Grange I found this knife, no makers name on it i'm afraid. The blade swivels in the head of the knife making it easier to produce cuts around curves.
Replacement blades are available and simply push into the cutting handel. Usage can be fairly high but the saving on having a straight curved edge is worth it.