For the first question, what if you use the upper bound of the shorter train (and the lower bound of the longer train)? Do you get an even smaller difference?
The volume is the space that an object occupies. It's a bit like area, but for 3D objects, like cubes.
To work out the area of a rectangle, you would multiply the lengths of its two perpendicular sides together. In a square, both side lengths are the same, so to find it's area you square the side length (multiply it by itself).
Volume brings this into 3-Dimensions: to calculate the volume of a cuboid, you need to multiply together the three side lengths; Volume (of cuboid) = a x b x c.
In a cube, all sides are the same length, so you need to do Volume (of cube) = a x a x a = a^3 (side length cubed). The maximum volume will come from cubing the upper bound of the side lengths. These formulas will* be given to you at the front of the paper or on a separate formula sheet, so you don't need to memorise them: you can just check the sheet.
EDIT: *Those formulas might be given to you, it will depend on your exam board though.