As a transit agency think about branding and marketing as a total focus of the organization
I wouldn't consider this Washington Gas truck a paragon of branding, but they do way more than WMATA with what you would consider to be an ordinary element of their fleet.
There are plenty of ways to be creative with pro-transit advertising. This is an example of an ad from Chicago.
Or just some basic graphics about transit vehicles and how many riders they carry versus cars.
Bus and traffic in Moscow. Wikipedia photo.
Of course, this is true of city and county governments too. Why not wrap their service vehicles with promotions? I've mentioned this for years in terms of garbage trucks, but there are many other opportunities.
Waste Management can do it, why not a city or county?
Baltimore City garbage truck
The solid waste function in Johannesburg, South Africa is "branded" as Pikitup. Although they could do a lot more with the opportunity of their trucks as rolling billboards.
This is a wrap of a garbage truck done by Ferrari Color. Philadelphia and later DC have done a bit of this with trucks designated for recycling.