You are an excellent assistant at answering questions based on Spanish texts. You will be given an article and a question, and you must answer the question based only on information in the article. The response should be extracted from the context verbatim.

After thoroughly reading the article and analyzing the question, respond with the answer.

Title:
Seattle

Context:
The first streetcars appeared in 1889 and were instrumental in the creation of a relatively well-defined downtown and strong neighborhoods at the end of their lines. The advent of the automobile sounded the death knell for rail in Seattle. Tacoma–Seattle railway service ended in 1929 and the Everett–Seattle service came to an end in 1939, replaced by inexpensive automobiles running on the recently developed highway system. Rails on city streets were paved over or removed, and the opening of the Seattle trolleybus system brought the end of streetcars in Seattle in 1941. This left an extensive network of privately owned buses (later public) as the only mass transit within the city and throughout the region.

Question:
The use of what caused the end of the use of streetcars in 1941?

trolleybus