Having spent many a happy hour in Cajon Pass, ( along with 1stcajon ), I would guess the geologic formations you are seeking, are the Mormon Rocks. While the Lower & Upper Narrows @ Victorville are notable, they go by in a flash & are not easily seen from on board. The Mormon Rocks are large, smooth sandstone formations that jut up from the ground at various angles, and are easily seen from on board for quite a while before rolling past. If your train was on the old North Track, you would have viewed them up close as well, while rounding the curves @ Alray, as well as Sullivan's Curve. Highway access is easy; simply drive North on Interstate 15 to Hwy 138 exit, head West 1 mile over all BNSF lines & under the EwePee ( ex - Espee ) Palmdale Cutoff, look on the right for the Alray Mormon Rocks. The formations @ Sullivan's Curve require dirt road driving to access; suggest using a rental car for this.
BTW, a similar but larger Geologic formation, the Vasquez Rocks, are located to the West in Soledad Cyn, between San Fernando & Lancaster, CA. Though Metrolink commuter trains roll nearby, these are best seen from Hwy. 14 Westbound. As well, another formation is located in Chatsworth, ( yes THAT Chatsworth ), and is easily seen from Amtrak & Metrolink trains on the ex - Espee Coast Line. Both Vasquez & Chatsworth Rocks were seen in many old Hollywood western movies & television shows.