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Marathon poster today, huh?

You got it right, trailers were originally shown after the film (and credits were shown before the film).

Now, get back to work!

I heard it was because they used to be screened on TV's on trailers being towed around by cars. This gave people wandering around the city a look and reached quite a large audience. Any truth to this?

maybe in Australia
No, I really don't know.

trailers are 2-3 minute advertisements Advertising is the promotion of goods, services, companies and ideas, most often through paid messages. Marketers see advertising as part of an overall promotional strategy. Other components of the promotional mix include publicity, public relations, personal selling, and sales promotion.

History of Advertising
In ancient times the most common form of advertising was by word of mouth. However, commercial messages were found in the ruins of Pompeii. As printing developed in the 15th and 16th century, the first steps towards modern advertising were taken. In the 17th century advertisements started to appear in weekly newspapers in England, and a century later advertising had become a popular thing.
..... Click the link for more information. made up of film scenes cut out of context. Trailers tend to appeal to specific markets, and if you compare different trailers for the same movie, you'll find that they also portray specific themes depending on their intended market. The term comes from their originally being shown at the end of a movie, though today trailers are usually shown before the film begins.

Your link didn't post Jus. A little FYI