"Sum If" questions are common in this forum.
There are several ways to do that in FileMaker:
You can set up a series of relationships using an added match field that matches only to child records of a specific route. Now sum can sum just the subset you want.
A summary field defined in the child table can be placed in a one row filtered portal. The portal filter specifies the route.
And unstored calculation fields can use the executeSQL function to compute these subtotals.