When local authorities offer subsidies for a route between two destinations, any new routes are obviously subsidized. However, I cannot tell if existing routes (say, a bus route from one town to another) are subsidized when a new subsidy is offered. Do I need to create a new route, new stations and a new route, or will my current routes be enough to get the subsidy?
A subsidy is awarded to the first company to move any quantity of the relevant goods from the source town/industry to the destination town/industry (ensure stations have the source/destination inside their catchment area!). Once awarded, all deliveries of that goods from source to destination made by that company will receive the subsidy bonus, this is regardless of the route, station or type of transport.
That is to say the subsidy effectively is awarded to the company, not any specific route.
Note that this rule changed in version 1.0.0 1 and differs from older versions, which bound the subsidy to a specific pair of stations.
So, if a new subsidy happens to have a source and destination that coincide with an existing route, and that route triggers the subsidy award before another company does, then yes, the subsidy will be applied to that existing route.