Hi Callum,
Thats unfortunate there is no scale given. All draftsman should indicate a scale on drawings. Thats a sectional detail drawing you show, sometimes these can be drawn full size, my comments above related to a full loco general arrangement drawings or say a drawing of the chassis only.
But I see figured dimensions on that drawing, so you need to get out your scale and measure the dimensions to see what scale the drawing is. I hope you are familiar with using scales and have a few, the dimensions are imperial so you need an 'imperial scale'
Eoin