(a)air is used to make inflate tyres.
(b)steel is used to make railway lines.
(a) Air is easily available and compressible.Therefore, it is used to inflate tyres.Other gases can also be used but their storage and use will be costly in comparison to air.So, air has a benefit of being cheap.
(b) Steel is a versatile material. If right composition and heat treatment method is used, it can be used to make many stucrures.Steel used for railway lines has a high tensile strength and appropriate hardness.It has resistance to internal and surface cracks.
a) Air is highly compressible.An air-filled tire is a light-weight construction element that also provides some damping and an easily adjustable amount of deformation to get the required footprint and required traction dependent on the surface conditions. Other gasses could be used instead, but air is easily available.
b)steel is used to make railway lines because steel is very flexible and has the ability to expand. During summer as you know metal expands.steel is used because when it expands it does not change it shape, it only becomes bigger making sure it is safe to travel by train.