Off the top of my head, bnsf50's Tehachapi Depot (<kuid2:39134:100039:1>) And the Central Station (<kuid2:63290:28020:1>) and Pitkin Station (<kuid2:63290:28018:3>) by elvenor all have very low loading platforms. I believe Bendorsey has some stations on the DLS that may fit the bill too. Although a few of these also use the custom Western Passengers, so you may need to change them to the default ones unless you want cowboys riding Amtrak.
Another alternative is to use one of the 'track only' station assets (just a track and attachment points for waiting passengers, really) and use a platform spline (the DLS has dozens) and a stand-alone station building.