I noticed that in the shield for Nebraska state routes, the covered wagons are not being pulled by horses. Instead it appears to be some sort of cattle instead of usual animals pulling those sort of buggies.
It is (as Rothman alludes) a Conestoga wagon being hauled by oxen, and reflects the fact that the Oregon/California Trail followed the Platte River for hundreds of miles in what is now Nebraska.
Wiki says it’s based on artist Robert Cochran back in the 20’s to focus on the state’s history, but Wiki is not creditable as far as facts as it’s made where anyone can post or repost and has no fact checkers to check the accuracy of its articles.
Though he designed the pioneer wagon silhouette that has been used on the state route marker in some form since (IIRC) the 1920's,
Cochran wasn't known primarily as an artist--he was chief engineer of the Department of Roads and Irrigation ("Department of Ruts and Irritation" to its detractors) and also, for a time, governor.
While Wikipedia does allow anyone to create an account and edit, and also permits anonymous (IP) edits, there is a degree of article curation and page patrolling, and it is generally trustworthy to the extent that information is cited to reliable sources.