Bus routing is just a variant of the vehicle routing problem (VRP), which itself is a generalization of traveling salesman problem (TSP) but with multiple salespersons doing the traveling.

There is an Excel sheet for solving such problems, and if one wishes to go further, the basic heuristic and metaheuristic algorithms are not that hard to implement either. http://people.bath.ac.uk/ge277/index.php/vrp-spreadsheet-sol...

With these keywords you are welcome to fall into this rabbit hole as deep as you wish. I promise to greet you on my way up (currently finishing my PhD on the topic).