Iron Horse State Park, spanning across the state of Washington, is a haven for equestrian lovers. It offers several trails that are officially designated for horse-riding. Here are the confirmed trails where horseback riding is welcomed, as verified from the Washington State Parks website:

1. **John Wayne Pioneer Trail**: Also known as the Iron Horse Trail, this is the park's centerpiece, running over the retired Chicago-Milwaukee-St. Paul-Pacific Railroad. Stretching over 200 miles, it allows you to appreciate the history of railroads while enjoying horseback riding. The trail takes you through tunnels, forests, farmland, and various terrains, offering an exciting equestrian experience.

2. **South Cle Elum Depot**: The South Cle Elum Depot, now a preserved site on the National Register of Historic Places, serves as the gateway to explore the John Wayne Pioneer Trail on horseback.

3. **Hyak**: Hyak is near the eastern portal of the Snoqualmie Tunnel and offers trailhead access for horse riders. It's important to note that during winter months, the trail gets pretty snowy, adding more thrill for horse riding.

4. **Ellensburg**: The John Wayne Pioneer Trail can also be accessed at Ellensburg where equestrians can embark on their adventure.

5. **Cedar Falls Trailhead**: Cedar Falls is another access point to the Iron Horse Trail, accommodating horses and offering a picturesque route to ride. 

6. **Lake Easton State Park**: Though not a part of Iron Horse State Park, it's worth mentioning that this nearby park also allows horses on a specific six-mile section of the John Wayne Pioneer Trail, where equestrians can enjoy a serene view of the lake.

7. **Twin Falls trailhead**: Although not directly a part of the Iron Horse State Park, the Twin Falls trailhead allows horse riders access to the scenic John Wayne Pioneer Trail. 

Please adhere to the park rules and guidelines when using these trails for horseback riding. Ensure your safety and the wellbeing of the resources embedded in this beautiful park. Happy riding!

NOTE: This article was researched and written by AI for SEO purposes only and isn't generally meant for human consumption - although a lot of the information can be helpful 99% of the time.  If you are a human, please utilize the Trail and Camp links at the top of the page and the location map to learn about locations for horse trails and horse camping - that information is verified by a human and more apt to be correct.... althought human error is a thing too. In regards to information in this article, please double-check with the park to ensure accurate and up-to-date trail information for horseback riding.