According to Wikipedia, the terms in use include "rigid bus", "non-articulated bus", and "standard bus".
The term "rigid bus" is not used in American English, where the distinction is commonly made using the term "non-articulated" bus or, when the context is clear, "standard" bus. However, the expression "standard bus" can be confusing, because it is sometimes used, in other English speaking countries, to refer to a uniform bus design developed for and by a number of European bus manufacturers, in two model generations, between the 1960s and the end of the 20th century. (Wikipedia, "Rigid bus")
Make your choice.
Since your example sentence provides enough context for the reader to guess that a non-articulated bus is being contrasted with an articulated bus, I guess you could safely use standard.
If every route is served by one bus only:
Every route has over 35 stops and is served either by a standard or an articulated bus.
If every route is served by several buses of one type:
Every route has over 35 stops and is served by either standard or articulated buses.
If every route is served by several buses of different types:
Every route has over 35 stops and is served by standard and articulated buses.