Alamosa and Salida are obvious narrow / standard gauge pairings. Other possibilities include:
Farmington, Maine. Sandy River & Rangeley Lakes / Maine Central interchange.
Johnson City, Tennessee. East Tennessee & Western North Carolina / Southern interchange.
Bellaire, Ohio. Ohio River & Western / B&O interchange.
Bellevue, Iowa. Chicago Milwaukee & St. Paul (Bellevue & Cascade) / CM&St.P interchange.
Denver, Colorado. C&S narrow gauge mogul next to a C&S 2-10-2, or the Leadville train in Union Station with a standard gauge train on the next track.
Montrose, Colorado. D&RGW. I have seen a nice view contrasting standard and narrow gauge ten wheelers here.
Mack, Colorado. Uintah Railway / D&RGW interchange.
Clifton, Arizona. Coronado RR / SP interchange.
Colfax, California. Nevada County Narrow Gauge / SP interchange.
Owenyo, California. SP.
San Luis Obispo, California. Pacific Coast / SP interchange.
Baker (now Baker City), Oregon: Sumpter Valley / UP interchange.
Santa Fe. NM. D&RGW / AT&SF union station.
If an earlier time (say pre WWI) applies, you could consider the Cripple Creek mining district in Colorado, or the Durango, Colorado yard before 1923 (small standard gauge power used on the Farmington Branch until it was narrowed in 1923), or a variety of locations in the Leadville area - C&S narrow gauge, Colorado Midland standard gauge, and D&RG mixed gauge.
Good luck!
Charlie Mutschler
-30-