The use of compound subjects, compound verbs, prepositional phrases (such as "at the bus station"), and other elements help lengthen simple sentences, but simple sentences often are short. The use of too many simple sentences can make writing "choppy" and can prevent the writing from flowing smoothly.
A simple sentence can also be referred to as an independent clause. It is referred to as "independent" because, while it might be part of a compound or complex sentence, it can also stand by itself as a complete sentence.