Rail wheel is not cylinder-shaped, It has a conical shape which enables the train to take the curves smoothly. Thus, If we install the rails perpendicularly on the sleepers, They will be under great stress because of the conical shape of the wheels (they could be dismantled out of the sleepers).
And so, the rails have to be installed with an inward incline to overcome the huge stress which is being produced from the wheels. (The other solution is very costy and complicated, It is to put the rails vertically on the sleepers but you have to redesign and strengthen the fastening systems).
In the last attached photo, There is a device called (the rail inclination device) which shows the incline level of the rail during the installation and maintenance processes.
The inclination can be made by the design of the sleepers( like the concrete sleepers) or the rail chair which is used with the timber or wooden sleepers.